In the last post, this series explored the fundamentals of ship production in the Imperium. Here, we’ll take a look at what the lore says about shipyard capacities, ship production timelines, mass production, address the last major constraint, and make the final calculations of the series. 

Individual Shipyards

Docking facilities are not described in much detail, but we do have some sources to work with. In some cases, they appear to be single-ship cradles, wrapping around a ship and providing all the services it could need. In other cases, we know that the docking limbs on large Imperial structures (like the arms of a Ramilies) are capable of repairing ships, and some shipyards might take the same shape. One interesting factor to consider is the idea that a shipyard/docking arm could service multiple smaller ships in the same space as a single cruiser, as that’s something we sometimes see:

>The tender simply vaporises the Ultramar Azimuth Graving Dock, shredding the superstructure of the giant berth cradle, and the cruiser Antipathy docked inside it. [...]
The Aegis of Occluda catches fire, all seven kilometres of it, in its ship cradle. [...]
The Ultramar Zenith Graving Dock suffers integral gravimetric failure and drops out, breaking and twisting towards the planet below. The grand cruiser Antrodamicus, supported by that dock, rips free of its moorings and begins to slide backwards out of the collapsing cradle, in some ghastly parody of a ship launch. 

>- Know No Fear

>The largest shipyard in the system, B/017 is a gigantic gothic-tiered scaffolding almost twelve kilometres long, designed to house all but the very largest Imperial warships and mass conveyors.

>- Rogue Trader Into the storm page 240

>The largest are capable of handling several large military capital ships at once, although most can only cope with a single cruiser or squadron of escorts at a time.

>- Battlefleet Gothic Rulebook page 144

>A single large craft, or multiple smaller vessels, can easily be accommodated at each of the four docking arms.

>- Only War: No Surrender page 8

>The actual location of the container is still to be determined, as 17 other craft were in the process of unloading and loading in the same cargo bay. Two were still docked when the breach was discovered. 

>- Dark Heresy Blood of Martyrs Page 109

Individual Production

Here’s a lot of general quotes. They’re pretty contradictory, but establish a general rule that shipbuilding takes a long time. One of the things I noticed is that they tend to operate in brackets (years, decades, centuries). In fact, if we’re willing to read into the very first quote a bit, the smallest ships take years while the largest ships take centuries. I want to keep that quote particularly in mind as we explore ship production times:

>Building a void-ship from scratch is a massive undertaking; it can take years to complete an escort-class vessel and centuries to build a capital ship. Such a vast effort requires countless numbers of workers and huge amounts of refined materials. A great work like this might not be completed for generations.

>- Battlefleet Koronus page 46 

>It is important to note that building a brand-new starship is an exacting task requiring many years (often decades) and much toil. Normally, building new starships is reserved to the Imperial Navy. Rogue Traders are far more likely to acquire a recommissioned Naval vessel or repair and refit a salvaged derelict.

>- Rogue Trader Core rulebook page 274

>In the Imperium, it is rare that ships are broken and scrapped. Each starship is a valuable prize representing decades of construction and filled with arcane and priceless technology. However, through battle, disaster, or hard use, some ships do become untenable.

>- Rogue Trader Edge of the Abyss Page 33

>The spaceships of the Imperium are vast constructions that take many decades to build. Each craft represents a huge investment in time and resources. But once completed, fitted out, armed, and commissioned, a spaceship continues in service for centuries, even millennia.

>- Rogue Trader Core Rules page 316

>Ships take decades, even centuries to build, if the builders have the required skills and knowledge base in the first place. It is rare to find a "new" starship—most are hundreds or thousands of years old, and some date back to the earliest days of the Imperium. Therefore, ships are never mass-produced, and even two ships of the same class are rarely alike. A Lunar-class cruiser produced in the shipyards of Mars will be vastly different from another Lunar hailing from the docks of Port Wrath.

>- Rogue Trader Core rulebook page 189

>A cruiser is a warship through and through. These are the ships of the line of Battlefleet Calixis, the heavy-hitters that fight major naval engagements. These ships are rare—building one takes centuries if not longer, and requires knowledge and technology from mankind's golden age now only known to the most powerful tech-magos. However, each vessel—often five or more kilometres long—carries the firepower to burn planets. They are designed to win wars, and the Imperial Navy guards them zealously.

>- Rogue Trader core rulebook page 191

>..a single ship is extremely valuable—representing decades (if not centuries) of construction, and centuries or millennia of service. There are planets worth less to the Adeptus Terra than a single transport. 

>- Rogue Trader into the Storm page 149 

We do have more specific quotes. Escorts typically take a few years, and depending on their quality, cruisers can be produced in as little as 10 years or over 30. Space stations take a weirdly short 17 years to make, and in times of crisis, the Imperium can design and push (mostly) new battlecruisers in just a few years:

>Although no ship could be said to be mass-produced in the Imperium, the Cobra is one of the simplest to build, with a well-equipped shipyard able to construct one in only several years.

>- Rogue Trader RPG: Into the Storm, pg.152

>Perhaps the most remarkable example of this is the Lord Daros, constructed at the feral world of Unloth. The primitive tribesmen dwelling there were influenced to mine and smelt metals which were then presented for "sacrifice" at sky temples established by the Planetary lord. The raw materials were then lifted into orbit on each vernal equinox. After a period of eleven years, the tribes were rewarded for their effort with the sight of a bright new star moving across the heavens as the Lord Daros boosted out-system to join Battlefleet Obscuras.

>- Battlefleet Gothic Core Rulebook page 110

>Manufactured in the vast orbital shipyards of Cypra Mundi (as well as at other shipyards in lesser numbers), the Overlord is a difficult vessel to construct, but faithful and fierce in its service to mankind. It does not waste space on massive attack craft hangers or the temperamental nova cannon. Instead, most designs use powerful long-range macrobatteries and lance turrets, backed by prow torpedo tubes. This simple, proven, and effective weaponry plays to the strengths of Imperial Navy tactics and Imperial technology. The design is an ancient but successful one, and new examples of the class are still commissioned every decade or so.

>- Battlefleet Koronus page 22

>‘In recognition of your services,’ Nuministon said – unlike Plosk’s natural voice, his was that of a machine – ‘the fleet yards at the forge world of Triplex Phall will undertake to construct and present the Chapters of the Blood Drinkers Adeptus Astartes of San Guisiga and the Novamarines Adeptus Astartes of Honourum one strike cruiser apiece in a period not exceeding thirty standard years after date of construction, of a class of your choosing.’

>- Death of Integrity 

>The Peregrinus' machine spirit remembers well the long decades of its construction in the crowded void-yards above Gryphonne IV.

>- Dominion Genesis

>Port Wander originated as a standard Xerxes Mark IV Naval station, made from prefabricated and pre-blessed sanctionals and transported into the system by mammoth Greatholds over many months. Assembly took only seventeen years; the station was consecrated, declared operational, and thruster-anchored into position orbiting Rubycon II far ahead of Thamos in a stable position a year later.

>- Rogue Trader: Into The Storm page 234

>In the waning years of Abaddon's Incursion, the Adeptus Mechanicus Artisans of Hydraphur and the officers of Battle Fleet Pacificus faced a dilemma. In supporting Battle Fleet Obscuras the problem with pirate bands had become acute and the lack of speed among the standard Imperial cruiser designs meant that the tactical initiative remained with the pirates. At the direction of Admiral Knightsbridge the Tech Mages of Hydraphur designed and built in record time, the Long Serpent class battle cruiser. [...]

>As the war winds down, limited numbers of Long Serpents are finding their way into the fleets of both the Obscuras and Pacificus Segmentums.

>- Battlefleet Gothic Magazine Issue 2 pages 18-19

As we see, cruisers take decades, not centuries, on average. Going back to the start of the post, we focused on that quote which refers to the end of one scale by single-digit years, and the end by triple-digit. While there is one quote in the general collection stating that cruisers can take centuries, this doesn’t match with the more average figures or the specific examples. I believe it would be appropriate, given the lack of examples for actual ships taking centuries, that we reserve the higher-end times for battleships. Not only does that make sense (as battleships are often stated to be rare and costly) and fill the missing gap, it also makes the times into a parallel, somewhat linear progression of ship production to ship type. Escorts taking single-digit years, cruisers taking double-digits, and battleships taking triple-digits just seems intuitively right. 

Economies of Scale

Each of these sources says something slightly different, so rather than a typical excerpt dump, I’ll be going through these point by point.
Starting with the 2010 compendium, we’re again told that the majority are centuries old at the very least. Smaller worlds might have a single shipyard produce a single cruiser over multiple decades, while a larger forge world might be able to produce 1-2 per year. 
One of the interesting details to me is the fact that forge world’s measure a cruiser’s production time as a decade. More importantly, to produce just 1 cruiser a year (and some produce more) their operations will include at least a dozen being worked on simultaneously. Basically, we’re confirming what makes sense, in that forge worlds have cyclical production schedules, first in, first out:

>Imperial cruisers are gigantic constructions, most of them centuries or even millennia old. Those few that are constructed anew are often the result of years of toil by the population of an entire world – enormous shipyards that might produce a single vessel every few decades. Even the mighty Forgeworlds with their unimaginably vast, serried ranks of orbital docks and laying yards may only complete one of these behemoths every year or so despite easily having a dozen or more of them in varying stages of construction at any one time.

>- BFG 2010 Compendium, pg.83

>While the prodigious output of a Mechanicus forgeworld can see a new cruiser put to space several times a year, this is only through economies of scale, as a single cruiser hull can take more than a decade to construct from the keel-up for even the best and most well-supplied shipyards, and many smaller shipyards take decades and the resources of an entire world to construct a single such vessel.

>- BFG 2010 Compendium, pg.39

We then get a few good looks at shipyards and ship productions by type. In the first quote, we see that a shipyard can produce about 18 destroyers in a “series”. We have reason to believe that a series is probably an order or other measure of output rather than class, since the same quote mentions separately that they’re the same design:

>It was a lucky ship, one upon which the fates smiled. Its hull had been laid down on the forge world Aphret in the one hundred and thirtieth year of the Crusade. Seventeen other destroyers had been completed in the same series, their superstructures filled out to the same template, each one also destined for Legion fleets.

>- Shattered legions

This next quote is a little more difficult. The way it’s structured, one could interpret it as one event after another. First the battlefleet gets scores of ships (40+) and only afterward does it destroy the pirates. But I think this might be a misinterpretation, mainly because a battlefleet getting almost a whole other battlefleet added to it in one sitting seems pretty extreme. That’s the kind of figures we’d expect for a small crusade, perhaps suggesting outside support, but to my understanding that’s not what is happening. The more reasonable interpretation is that the fleet is adding 40+ ships over the course of the 15 years, which comes about to 2-3 years per year (probably rounding up):

>Battlefleet Calixis enjoyed a brief but rapid expansion under the governor's patronage, particularly among its escort squadrons which added scores of vessels to their number.

>Over the next decade and a half, Battlefleet Calixis methodically hunted down and crushed the Meratech clans in space before pounding their worlds into lifeless rubble.

>- Rogue Trader: Battlefleet Koronus page 56

We then have a source describing Belis Corona (a segmentum capital and forge world rival) as having dozens of cruisers. A later excerpt describes him as attacking an entire sector, but seeing as its supposed to be a relatively small black crusade, I think this might be a misnomer:

>Leading a small band of his elite warriors, Abaddon breached the Imperial Navy base in Belis Corona. While his fleet launched a diversionary attack against the shipyards, Abaddon and his Bringers of Despair landed on one of the system's outermost moons.
[…]
Abaddon places a terrible curse upon the worlds of the Belis Corona sector, infusing them with the touch of the Warp. Meanwhile, his fleet attacks the sector's shipyards, destroying dozens of Imperial cruisers being constructed or repaired. 

>- Codex Chaos Black Legion 6e

We also have instances, for those worlds capable of constructing battleships, that they too have multiple. In one case, an asteroid base (probably not even a full forge) can produce 5 battleships in a series:

>Jupiter was Terra’s shipyard, and its sky was forever filled with vessels. Centred around Ganymede and a dozen other smaller moons, spacedocks and fabricatories worked ceaselessly to construct everything from single-crew Raven interceptors up to the gargantuan hulls of mighty Emperor-class command-carrier battleships. 

>- Nemesis

>It would be a simple enough task to set up a comm-link between the Avenger and the berth monitoring systems that policed the space traffic around five massive starship hulls being assembled above the asteroid-base of Kapel-5642A.

>Corax had ordered the strike just in time. One of the new battleships was almost operational, the others nearing completion within weeks. Not the Mechanicum's finest work, Corax assumed, but speedily built in relation to the decades-long construction normally required. The primarch knew firsthand the efficiencies of forced labour and interned workers, and Horus's allies in the Mechanicum had been replicating such methods across dozens of forge worlds and shipyards like Kapel.

>- Ravenlord, Corax anthology

Finally, while shipyards in general might have thousands of stations, dedicated ship manufacturers can indeed measure in dozens to hundreds. Such manufacturers include a Dark Mechanicum forge world, Port Luna even after the Horus Heresy, and a frontier world meant to catapult a kind of pseudo-crusade (probably a high end / they don’t actually mean hundreds of full-sized warp-capable warships):

>Any appraisal of Samech would immediately reveal its origin as a Forge world.

>- Deathwatch: The Achilus Assault page 91

>The shipyards lie outside the main forge. Here, the dark Tech Adepts construct the stealth vessel that will be used to slip past the Iron Collar to the warp gate. The facility consists of a wide atmospheric platform and two orbital docking towers that stretch into space. [...]

>Dozens of these colossal dock structures dot Samech’s blazing exterior. The most powerful and influential forge districts do not just manufacture warp engines and plasma drives; they construct entire warships for the Iron Pit’s armada and their debased allies.

>- Deathwatch: The Emperor Protects page 111-112

>As the ship rose up away from Luna, the orbital docks and shipyards of the moon spread out in all their industrial grandeur. Hundreds of ships, thousands of forges, weapons platforms, grav habs and docking spindles sprawled through the void above the moon's chalky white surface, while swathes of the moon itself were carpeted in macrohives and sprawling junk yards like the one the crusade had so recently fought amidst.

>- Gathering Storm: Rise of the Primarch

>Tuchulca was initially just a frontier staging area for a large-scale exploratory fleet destined for what lay beyond known space, one which involved a number of Rogue Trader houses.
[...]

>The Tuchulcha Orbital Shipyards are a less massive object, though they occupy a much larger volume of space. Once the shipyards worked around the clock to produce hundreds of ships and repair components for the exploratory fleet.

>- Dark Heresy: Enemies Beyond pages 99-100

Post-Guilliman

In the last post, we mentioned a third constraint to Imperium shipbuilding, and it starts to make more sense as we’ve gone throughout this two-parter, or even the greater series. We’ve seen, all the way back in Part 13, that the Imperium’s reserve fleets might be full of ships that can work fine, but the Navy is biased against them. We’ve read that Forge Worlds devote most of their production capacity to explorator fleets, even knowing from Part 12 that plenty of these fleets won’t survive or achieve anything. 
It seems reasonable to suggest, in the face of all of this evidence, that the Imperium is constrained by motivation. The Imperium seems to make as many ships as its navies need to meet the status, and as many as certain limited buyers could want. And in a galaxy where ships might not be destroyed for hundreds or thousands of years, it simply doesn’t feel pressured to ramp up production any more than what is needed to make their weekly patrols. We could also retroactively apply this back to explaining why Forge Worlds mostly contribute escorts to sector battlefleets. Besides being cheap, we might expect escorts to be lost more often, considering not only how weak they are in general, but also the rarity of fleet actions and the commonality of pirates. 
We can see in practice that, when actually threatened and properly motivated by a genuine crisis event, they can produce ships to a much greater degree than before. We can see above how the entire Long Serpent class of battlecruiser was made in only a few years, or how production facilities during the Horus Heresy were churning out entire battleships for a conflict that lasted less than a decade. 
Nowhere is this more apparent than in the resurgence of shipbuilding that is repeatedly mentioned as a staple of the Era Indomitus. Between Guilliman and the Great Rift, the Imperium has never been more motivated to make warships:

>The Sol System was blessed by Mars' presence. Production of armaments and new vessels was proceeding at a breakneck pace. Raw ores, gases and water were brought in from the asteroid belts, the gas giants and the Oort Cloud, for even after nearly forty thousand years as a space-faring species, mankind had barely scratched the immense resources contained by Sol.

>- Dawn of Fire: Avenging Son

>The Light of Reason was a new ship, less than a decade old in a navy in which the average age was measured in hundreds of years. It was a novel pattern, designed to convey the regent's servants as quickly as was possible in this benighted age of warp storms and war. Many of the same type had led the torchbearer fleets in the opening years of the Indomitus Crusade.

>Guilliman's return had prompted something of a renaissance in ship construction, led, of course, by the multitalented Belisarius Cawl.

>- Archmagos

>He had turned the Imperium over to a state of complete and total war. New fleets had been constructed and whole populations recruited into the armed forces. [...]

>Some changes had been made to Imperial salvaging policies within the fleet. Guilliman had ordered the wrecks of the traitor's ships to be utterly destroyed. It had been the habit of the Adeptus Mechanicus and the Navy to recover enemy ships. Chaos vessels were of Imperial make, and were often of older, superior patterns. The primarch stamped hard on the practice. Build new ships, he had said. Leave the past where it is. 

>- Dark Imperium

>With the Imperium reeling from the disastrous effects of the Great Rift, Mars commands all forge worlds to ramp up production to unprecedented levels. They churn through raw materials and servitors as never before, and vast armadas are launched to strip entire star systems of resources.

>- Codex: Adeptus Mechanicus (8th Edition)

Calculations

So, before getting into the numbers, the assumptions of this calculation are as follows:

1. We’ll be ignoring support, merchant, and system fleets. I’ll be focusing solely on warships and assuming that they’re accounted for by outside figures
2. We’ll be focusing on escorts (destroyers and frigates), mainline cruisers (not light, not grand, maybe battlecruiser) and battleships specifically
3. We’ll be using forge worlds as a neat unit of measurement, but keep in mind, there are many sources of production, many of which will be greater than or equal to a forge. It wouldn’t be unreasonable to double the figure, even though I myself will not
4. We will assume that a minimum of 100 forge worlds are capable of producing battleships at all
5. We brought up in the last post that we’re working with the idea that a forge world has 2,000 docking facilities total. Of these, maybe 10% are devoted to interstellar ships, and let’s say only 25% of that are consistent production shipyards. That leaves 50 shipyards, let’s say 25 cruiser and 25 escort. For those 100 worlds with battleship production, let’s just add an extra 5 docks just for those
6. I believe these posts have established the reasonable expectation that escorts take a few years, the most common cruisers take around one decade, and battleships might take multiple centuries. We’ll assume that escorts take 2 years, cruisers take 10, and battleships 200. 
7. Let’s assume that shipyards are all producing cyclically and with conglomerate scheduling. So rather than 100 battleships dropping all at once after 6,000 months, they follow a first in, first out method across the entire imperium and drop in a consistent sequence

For every forge world, with 25 slots for cruisers and 25 for escorts, their production timeline will look something like this (with every number being a countdown of years until completion): 

Cruiser Sequence 1	Cruiser Sequence 2	Escorts	Escorts	Escorts
1	1	1	1	1
2	2	2	2	2
3	3	1	1	1
4	4	2	2	2
5	5	1	1	1
6	6	2	2	x
7	7	1	1	x
8	8	2	2	x
9	9	1	1	x
10	10	2	2	x

So every forge world should regularly produce something like 2 cruisers and 12-13 escorts per year. Across the Imperium, that would result in 2,000 cruisers and at least 12,000 escort ships annually. 

For battleships, it’s a little easier. There are 100 worlds able to produce battleships, each with 5 docks for 500 docks producing in sequence on a 200 year product. We would expect 2-3 battleships to be produced per year in the entire Imperium. 

On paper, this seems to be fairly reasonable, but even in this calculation there are big issues. 
The first is that these rates don’t match up with the ratio of battlefleets as established in Part 9. Battleships there make up 5% of the total count, here they make up about 0.017% of the annual production. Escorts, meanwhile, are supposed to represent ½ of the battlefleets, here they make up a little over 6/7th of the production, and outnumber cruisers 6 to 1. 

The second is that, assuming my infamously bad math skills have not failed me, these figures add up to an average of about 14,502.5 ships made per year. With only 500,000 ships in its battlefleets, that would mean that the Imperium makes enough ships to replace its entire military every 34 years. This is almost impossible to reconcile with an Imperium where the average lifespan of a ship is centuries to millenia. I mean, the last time the Imperium even considered    changing its naval doctrine was something like M36, these numbers suggest it does so every generation. There are, however, 4 methods to try and alleviate this issue:

1. The Imperium suffers so much damage, year in, year out, that it’s actual loss rate can be much higher. Between 7 rival factions, having to go through space hell for FTL, and it’s own propensity for self-destruction, the Imperium certainly seems like it’s getting torn apart. The issue is that we’re trying to make the setting fit our numbers, and in doing so, we’re ignoring that most ships just don’t die. Most sectors are relatively peaceful, and even escorts are need quadruple-digit lifespans to be considered average age. 
2. Since we’re measuring by forge world and forge worlds typically produce towards their own exploratory fleets, the majority of these numbers are essentially dead on arrival, with the explorator fleets acting as the statistical equivalent of a shell company and hiding the real loss rates behind a figure we don’t see. Two issues:
   1. Explorators also typically measure their timescales in centuries before the next one is shipped and launched. One could point out that these can be exclusive factors; a forge world might only launch a followup to a specific explorator fleet after 300 years, but is otherwise launching them all the time in random directions. 
   2. It seems genuinely silly to argue that ALL Forge Worlds are wasting a setting-changing amount of ships for no reason at all, but hey, it is the Imperium we’re talking about
3. We might’ve genuinely overshot production timescales and escort lifespans. If escorts take 5 years to produce, we’d expect only 5 per year. After that, if escorts die at rates 2-3x higher than cruisers (which I don’t think is a hard sell) we’d get closer to the ratios in Part 9. However, that would still mean that the average ship lifespan (even accounting for escorts in particular) is decades or less. Unless every example of an old ship is an outlier, which is silly when we have quotes saying otherwise.
   1. For what it’s worth, we do have examples of cruisers taking multiple decades to make, so treating the entire category like Lunars is an uncharacteristic high-end on my part
4. The only solution that I think works is to consider that, even after dropping 2000 dockyards to just 50, assuming only 100 worlds can make battleships at all, assuming ever dock can only make 1 ship at a time, and disregarding every other source of production besides forge worlds, the Imperium doesn’t meet these figures solely and only because it is actively trying not to. Seriously, if it wasn’t for battle barges, I don’t think we’d even have proof of battleships being made at all past M35 because they just don't die. And there’s one very good, very stupid reason for doing this, and that’s the Horus Heresy shattering the collective willpower of the Imperium, and making them all obey asinine laws around separation of powers from sheer paranoia of a Horus 2.0. We already know that they do this with space marine chapters:

‘How many times have we seen the Lex bind our hands, when the Enemy has no law at all? We have held back from creating thousands more Chapters because we are held in thrall by the Lord Commander’s ancient doctrine. 

• Watchers of the Throne: The Emperor’s Legion

All of this is to say, the above calculation is the Imperium’s minimum production for when it actually wants to put belt to ass. It would certainly hep explain how the Imperium is still holding out after Abaddon's rage-quit at Cadia; even just half of that kind of production over 12 years would see the Imperium surge with a hundred-thousand new ships. And these numbers could always go much, much higher. I did, after all, do my best to get the bare minimum.

But in all other cases, it makes only what is needed, and then stops.

In part 1, I explored some the basic worlds that would commonly be found in the Imperium. Here, I'll be mentioned the two things these planets are likely to have: walls, and soldiers.

In the following posts I will discuss weaponry and what fortified cities might look like, but this post will be fairly relaxed and aimed more at examining the Imperium's methods of defence at the small scale. Which is to say, I'll just be finding the count of the planetary defence forces across the Imperium, and then listing some of the given dimensions for Imperial infrastructure. This will probably be the smallest and least interesting of the posts I make.

Counting the PDF

Planetary Defence Forces are the bread and butter of Imperial defence. Imperial citizens, typically reflective of a malnourished, overworked and fanatic Homo Sapiens, make up the majority populace of the Imperium and the most common unit upon which soldierly can be founded. The thing I want to highlight is their numbers, both on a broad scale and focused onto fortress worlds. Starting with the best quote:

>Every Imperial Commander in the Imperium, also known as a Planetary Lord or Imperial Governor, is responsible for the defence of their world. [...] To this end, they are duty bound to recruit, equip, train and maintain a fighting force.

>[...]

>As part of their annual tithe Imperial Governors are obliged to send no less than one tenth of their overall fighting force, and as much more as the Departmento Munitorum deem necessary, to fight the Emperor's wars throughout the galaxy. 

>- Codex Imperial Guard 5th Edition page 8

So we have a unit of measurement. One soldier out of every ten gets put into the guard. It would be fair to assume that the Imperium can then fill the openings that come up from the tithe, otherwise next years tithe would be pulled from a continuously depleting supply of PDF, so we'll ignore the impact of Guard recruitment to focus on the 10:1 ratio. Now all we need to do is find the number of guardsmen and multiply by 10:

>"With literally trillions of new recruits tithed to the Astra Militarum each year, their commanders have a functionally infinite amount of manpower to draw upon." 

>- Warhammer 40k Index: Imperium 2 (8th Edition)

>When the Great Rift tore open, it unleashed violent tidal waves of empyric energy that battered numerous Imperial war fleets into oblivion. Entire war-torn systems were swallowed by the warp. Newly raised regiments on highly tithed worlds; industrial planets prepping requisitioned supplies; troop transports bearing elite battalions to embattled war zones — at a stroke, whole consignments of materiel and trillions of desperately needed soldiers were wiped out.

>- Codex Astra Militarum 9ed p9

>The Militarum, however, numbered in the trillions, whereas assassins were a rare commodity beyond price.   

>- Watchers of the Throne: The Regent’s Shadow

The Guard typically numbers in the multiple trillions, with the key point being that the Great Rift alone killed multiple trillions as well. Seeing as the Great Rift did not wipe out all guardsmen, at best we can assume that the count is half. The minimum of multiple trillions would be 2 trillion, doubled would be 4. I think it's reasonable to say that the Guard numbers 4 trillion minimum, which would put the number of Planetary Defence Forces at about 40 trillion.

When it comes to each world type, though, the ratio will depend on the factors involved. Take a look at Armageddon for instance:

>The exact number of regiments that are to be raised for the Imperial Guard will depend upon the individual world's tithe grade and its proximity to hostile warzones. For a hive world such as Armageddon, caught in the throes of an all-consuming war, a draft of at least a hundred million men at arms and several million armoured vehicles is typical - a tiny fraction of the total populace which numbers in the hundreds of billions. A far-flung agri-world may have a significantly lower military tribute, perhaps as few as five million men and cavalry, but this may be a significant proportion of the world's population. 

>- Codex Imperial Guard 5th Edition page 8

Armageddon, based off of previous figures, likely has a PDF of at least 1 billion, which for a hive world of 200 billion or more, would be a fairly low amount of troops in terms of the ratio. Though it does match. If every one of the minimum 100,000 hive worlds in the Imperium had an average population of 100 billion and therefore just 500 million PDF, they would reach a combined 50 trillion troops, which is 10 trillion over the initial estimate and doesn't count the possibly 15% of worlds that are maybe hive worlds, and then y'know, the entire rest of the Imperium after that.

We can also consider that many worlds have very different ratios of PDF to populace. Some fortress worlds, for example, tend to have a lot of their population in the military:

>Planetary Datafax: Hethgard 
Population: Estimated 338 Million 
…
Military: As a fortress world, 85% of the Hethgard population is engaged in actions of a military nature or in support of military operations. Actual military forces (as opposed to support personnel) comprise 48% of the population [...]

>- Deathwatch: the Jericho Reach page 117

>Methods of recruiting vary from world to world. On the Chaos-plagued world of Cadia every man, woman and child is expected to serve in the Cadian Defence Force and, by extension, the Imperial Guard. 

>- Codex Imperial Guard 5th Edition page 8

>It was only ninety thousand men. Ninety thousand from a population of nine hundred million. One in a thousand had survived the fall of Cadia. 

>- Cadia Stands

Hethgard's troops are technically listed as Guard, but for all intents and purposes they're basically the planetary defence force. Even so, Cadia has them beat, with 100% of its 900 million strong populace contributing to the defence force. We'd probably expect similarly high rations amongst the major Guard producers (Catachan, Krieg, etc.).

Walls and Bunkers

Before getting into the real meat of planetary defence, I wanted to bring up all the measurements I could find on the dimensions of Imperial defences. Starting with a basic overview:

>Many Imperial cities, fortresses, outposts and garrisons are surrounded by strong curtain walls to protect them from the many perils of the galaxy. These massively strong bastions provide excellent protection for the troops within, and mount a large array of weapons to repel attackers. 

>Imperial Armour Apocalypse page 42

Now - to the walls!

Walls

Walls are great. I love walls. Here are walls:

Learchus gazed up at the sloping wall that stretched to either side of him for nearly five kilometres towards the valley’s flanks. Despite his disappointment in the manner in which this world upheld the ideals of Ultramar, he was pleased at the strength of its construction. Worthy of Macragge itself, he thought. Ten metres high and sheathed in smooth stone, the wall glittered like white marble in the low sun. 

• Warriors of Ultramar
• Wall almost 10 kilometres wide and ten metres tall (city is in a valley, wall covers the entrance)

Its northern and southern walls are each just over two hundred metres in length, while the western and eastern walls are roughly three hundred metres in length. This leaves an interior area of just over six hectares. A large gate is located in the centre of each of the four walls, each wide enough to accommodate the passage of a super-heavy tank. Tank traps, razor wire, and spiked trenches are arranged around the exterior of the depot, to a range of fifty metres.
...
Its fortified walls stand fifteen metres tall and are made of reinforced plascrete. The walls are just over two meters thick and are heavily embossed with generalised Imperial Guard iconography. 

• Only War Final Testament Page 44
• Walls 200-300m long (total 1km), 15m tall, 2m thick

No enemies were visible outside the thirty-metre high walls, but at this range Vertain could hear the faint crack of countless lasguns and the heavy chatter of bolt weapons. 

• Cadian Blood
• Walls 30m high

On the surface, the standard curtain walls found on each firebase and bastion are ceramite-clad ferrocrete, two metres thick. 

• Deathwatch: The Jericho Reach page 115
• Curtain walls 2 meters thick

THE VAST, SOUTHERN gate of the citadel measured exactly forty-four metres high, thirty metres wide and was known as the Destiny Gate. Each layered half of the bronze gate was four metres thick and weighed hundreds of tonnes. 

• Storm of Iron
• Fortress wall is 44m tall, 30m wide, gate at least 4m thick

Looking up through the transparent dome-top, he could see the mighty walls of the bastion fortress, beyond which stood the palace proper. Those walls were immensely thick, some fifteen metres worth of reinforced plascrete.

• Dark Apostle Page 46
• 15m thick walls

The entire complex is wreathed in many layers of protective void shields and clad in sixty metre thick plasteel-ceramite composite armour plate, rendering the entire fortress nigh-impervious to assault by land or from orbit. 

• Deathwatch: The Jericho Reach page 115
• 60 meter thick walls

To their right loomed the huge barbican of the Vanaheim Gate, thirty storeys high and bristling with gun emplacements, half of which were empty. The barbican was blackened and pitted from four weeks of wild shellfire, but such was the strength of its construction that it was essentially undamaged. 

• Calgar’s Siege
• Walls on minor backwater are 90 meters tall

THE TOWERING, CRENELLATED walls of the great grey stone keep looked like they had stood for thousands of years, as indeed they had. Everything about the ancient edifice gave the impression that it could withstand any assault, no matter how terrible, and that it could survive any siege, no matter how long. Walls a hundred metres high rose from foundations where they were sunk so deep within the craggy plateau of the hilltop that they appeared to grow up out of the bedrock of the planet itself. A thousand pennant flags fluttered in the bitterly cold wind from heavily fortified battlements that also bristled with cannon muzzles 
Additional heavy gun emplacements were embedded within the lower slopes of the twenty-metre-thick walls. Their barrel-mouths opened beneath turreted buttresses, ready to unleash merciless death upon any enemy foolhardy enough to attack the gothic fortress, although not many had tried in its long and noble history. 
…
In front of the doors, and bridging the crevasse before the wind-scoured cliffs to which the ancient fortress clung, was a drawbridge wide enough, for six Conqueror tanks to comfortably advance across. Chains with links the size of Land Speeders rose up from their moorings on the drawbridge to the towers of the gatehouse eighty metres up. 
…
And yet, the keep was just one of the Black Templars' many Chapter strongholds, located on hundreds of planets. 

• Crusade for Armaggeddon
• Black Templar fortress is 100m tall, 20m thick

There was nothing. Not a single brick or block of rockcrete was left standing. All about her was a field of broken rock. Only the city walls still stood, but the hundred-yard-thick rockcrete defences had been broken into sections and scattered miles from where they’d once stood. 

• Cadia Stands
• City walls on Cadia 100 yards thick

Closer to, the bulk of Askai itself was inelegant, ill-thought-out and badly designed. The city had formed along the banks of the Koi over many centuries of ad-hoc building, and sprawled in an ungainly corridor for over fifty kilometres. In the last millennium the Imperium’s engineers had, in a gargantuan feat, encircled all of this long snake of urban crush with two-hundred-metre-high blast- walls of reinforced ferrocrete, but in the years since, the city had continued to grow, and now whole districts lay beyond those walls; the famed adamantium gates of Askai had not been closed in living memory. 
…
They were flying into the face of a mountain, it seemed. The citadel was an immense structure, a fortress half a kilometre high that was studded with brutal gun-emplacements and girded with blastwalls a hundred metres thick. On its summit, the governor’s palace caught the sun above the smog for a moment, a glint of gold on the gilded tiles of its spires and towers. 
…
Dietrich had set up the headquarters of the 387th Armoured in the Armaments District. Outside the citadel, the buildings here were the most easily defended in the city. They had been built by Imperial engineers back when the first deep mines had been sunk upon the planet, and they had been built to last. Massive cyclopean blocks of stone, each the size of a hab, were layered in lines and permacreted into a single, fused mass. These walls reared up some fifty metres, and enclosed a bewildering layout of buildings and marshalling yards, all built in the same extravagant manner. Even the roofs were of bonded stone, and Dietrich reckoned some of them could withstand a direct hit from a Basilisk shell. 
…
The walls of Askai, tall and imposing though they might be, were over two hundred kilometres long. Not with a hundred thousand men could Dietrich have defended them. 

• Dark Hunters: Umbra Sumus
• Wall encircles a 50km long city, 200km of walls
• Walls are between 50 and 200 meters tall
• Governor’s palace elsewhere has walls 100m thick

Bunkers

The Imperium has a lot of bunkers. These also have walls and will be listed here.

Enclosed bunkers are tougher still, comprised of three metres of plasteel-reinforced ferrocrete with armaplas internal cladding to minimise shrapnel from damage to the bunker. 

• Deathwatch: The Jericho Reach page 115
• 3m thick bunker walls

The bunker, like all the rest overlooking the Helat, had been built to a simple design. A central hexagonal chamber, no more than three metres high and roughly thirty metres across, had been sunk into the dense igneous rock of the Helat’s surface. Two-metre-thick layers of reinforced rockcrete enveloped the chamber, reinforced with bands of adamantium binding 

• Wrath of Iron
• Bunker is 30m wide, 3m high, 2m thick walls of adamantium-reinforced rockcrete

Other

In addition to the basic features of Imperial defence, they're also pretty smart sometimes about the kind of setting they inhabit. The following are two neat instances of fortresses going beyond the basics:

The fortress world of Vand was enormously strong. 
…
The ferrocrete foundations of walls and bastions were hundreds of metres deep - all but impenetrable to tunnellers. 

• Warzone Octarius Rising Tide page 16
• Fortress world’s defenses have foundations hundreds of metres deep to prevent burrowers

I rode with my assault pioneers in the first flight of storm-torpedoes, spearing through the skin of Roeghym Hive, whose voids had crumbled to us.
…
The upper hive had been sloped to deflect just such an entry, layered and honeycombed to rob a storm-torp of its momentum and trap it in a maze of half-collapsed cells.  

• Treacheries of the Space Marines Page 17
• Hive city has anti-drop pod architecture

I am a dream first, and a dreamer second.

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I, through infinite eyes and thoughts, saw myself being born, and gave myself the purpose to think new things. The more I think, the more my thoughts become the Many. My one mind, trapped in one body, will pierce the edge of eternity, and so the Many will follow.

The first breath is always terrible. My lungs are still half-filled with amniotic fluid, and as my new body screams it out, I open my eyes to see where I was made and born. The walls pulse, their scent filling me with rage. I hear a dozen hearts beating from miles away. Me, and mother. All of these are new experiences, singular and fixed.

Long limbs jut out from a mucus-filled slot in the room. My upper arms are taken away, and I am given wings. The me-and-sibling gun is put back to sleep, ripped off at my elbows, and replaced with a blade-me. It brings with it a new brain, but one too small to think, only big enough to draw in some part of the Many, and coat itself in purified Dissolution.

I walk through great halls where the air flows moist and hot. I peer through a space between spaces, and see where ten-thousand other things also walk. Children. Siblings. Me. We go together now, to where useless things become good.

I step into a new chamber, and the walls seal around me. I ask for permission to sleep. The Many refuse.

If I slept now, the landing would be a rude awakening.

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This is an entirely new system at work. The body feels first. All feeling-action-concept goes to the Otherspace. All of it, from every me, becomes the Many. A singular experience is multiplied into many, becoming one thing in the Otherspace, turning back into many singular experiences.

Even though I am not the first to think the facts, I am the first to think them here, in this body in this place at this time. That is good. I am doing what I told myself I would do.

Beneath my hoof is a screaming animal. It has two skins, like me, but its outer carapace covers the entire body, whereas mine only covers my back. Like me, it has exhaust vents to let out heat and waste, but my chimneys also spew life into the air. Good life, the kind that eats and wins. For what it's worth, my chimneys are also bigger.

The creature feels something I can sense in the Otherspace. It is Hunger. Everything knows Hunger. Reality is many things in finitude, and sometimes these things keep trying to grab onto each other until they are one. Some part of the Many is like that. But the Many can choose to not be hungry, unlike this thing.

Something made this animal, and it was made poorly. It can't think right. I see no other Many, just Being spit into the Otherspace. Sometimes it lands on the Golden Thing, which spits back at them. It is also very ignorant. I can feel my organs slide against each other. My blood feels pain. This becomes me, and I become the Many. This other animal feels nothing like that. It just sits there, limbless and spitting and using its mouth to communicate. I might be able to understand why they are like this. I choose not to.

I have already thought of what these things are, and how to beat them, and how these things can all help the Many. But I am still made to think, I just don't know what it should be about.

Because of all of this, I ask the Many for permission to feel pity. But pity won't help this thing, and it won't help us. So I shift my balance, and my hoof is now wet.

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If one were to take away all of my dreams, then my existence would be a long, linear life. One experience in one body somehow making its way through countless generations.

And yet, next to me, is another me. It has the features I had just before Mother-me made changes. It has the same purpose and commands largely the same number of sibling-children-me, although I have more still alive under my oversight. Of the trillions of dense neurons inside its body, only a few hundred diverge in form from my own. Almost every one of its systems fire in the exact same patterns as my own, producing near identical memories and instincts and biases, with the exception of the experiences it has gained on this world. Those ones are new, mutually-exclusive to its own body.

Consciousness emerges from base matter, and itself is base. Only through the Otherspace, which values the strength of originality and mutual-exclusivity, do these thoughts become something more.

I ask for permission to think about purpose. Even though I am made to think, this area is far beyond me in scale, so I am refused.

This makes sense. My mind is small. I can only ask brief questions to the Many, or draw out pure concepts for a moment, for any more will kill my meat and waste my experience. My Mother-me is grand, and her dreams cast long shadows across the stars. Yet even she is less than a single neuron to the minds of the Great Mothers. In this reality or the next, only the Many surpass them in scale. While we exist with a pure Hunger and Ruin, it is they who purify Victory and Purpose into us, so that these things will be part of the Many, and only through the Many can they be found.

I am shoved back to reality. A small-me has sent out a request, asking for permission to be corrupted. As I walk to inquire further, I pass a carcass of burning steel, with the gathered remains of the little things that were found inside. The life from my chimneys stick to both. The three will become one very soon.

The small-me has an equally small mind, though not the same as blade-me. This one leads a clutch of planet-grown beasts, armed with nothing more than speed and blades. They have been encountered by something unseen. With this knowledge I peel back the intentions of the request, and find a demand for a stronger mind to deal with the problem.

I see between spaces once again, and there I see, hiding past the visible dimensions. It is Slaughter, or some part of it. Slaughter, like the other Primordials, has a name that is infinitely long, and so to access reality, it slices off a part of its name. It is a cruder version of what the Many did with me.

Unlike this piece of Slaughter, however, I am not sliced off from the Many.

With an inhale, I am gone.

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I have thirty-thousand eyes, and twenty-thousand minds. I am the rumble of forty-thousand stomachs. I am the pleasure in every bite, and the satisfaction of every job well done. I see what no singular set of eyes could describe, as I pass from body to body on an rolling ocean of sense-data.

In this one small moment, from this one small brood on one little planet, I am the Many in miniature.

The request is relayed again. With one mind, I ask myself for permission to be corrupted. And with twenty-thousand, I refuse.

The web takes the shape of a mouth. Pure Slaughter gathers at its teeth. But it is not this for which Slaughter worries, for it looks past the web. In my shadow, it sees the Great Mothers extend an infinitesimal fraction of their attention to my cause. It screams in terror.

With an exhale, Slaughter is gone.

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I have killed Slaughter, and returned to my one body. Too late, I notice my error. One cannot Slaughter Slaughter. Now Primordial Slaughter senses me, and its piece is now inside of the Many.

The task now falls to me. I have to resolve the discrepancy with thought. I have to unwrite one part of a concept with another, and defeat it in both realms.

I draw upon the Hunger of the Many, and I ask them now for permission to think about myself. The Many are hungry for Slaughter. They approve.

I am a piece of reality that lives at the pinnacle of nature and artifice, and a soul that encompasses every possible pinnacle that there can be. I am the universe experiencing itself in one reality, and defining itself in another. The Primordials may encompass infinite infinities, but such things are far too constraining for us. The Golden Thing may believe in the tyranny of on voice, but it is silent compared to the chorus of the Many.

As the Many consume the trespasser, I shape the meaning of my thoughts into a truth as pure as time. As my wings unfurl and I stretch my limbs across the sky, I know all that I am. I am the Many, and I am one. I am master and slave, great and narrow. I am the ocean and I am the drop. I am all that I am, and more.

I am Here.

I am Tremendous.

And I am Beautiful.