u/HoodsFrostyFuckstick

Agamemnon (sorry for spelling error in the title, can't edit that) and his army from the movie Troy (2004) spawn in Rohan through a portal 10 miles from Helm’s Deep and are tasked with capturing Helena who is hiding within the fortress. He has 50,000 Greek soldiers + an additional 2,500 elite Myrmidons led by Achilles. Named characters from the movie like Menelaos, Odysseus, Ajax and Patroclus are also present. Their gear and weaponry is bronze. Unlike in the movie, Achilles is now willing to fight for Agamemnon the whole time.

Win condition for the Greeks: Kill all the defenders, capture Helena and return with her back through the portal.

Helm’s Deep is defended roughly as in the movie. Theoden and his household, 300 armed Rohan peasants ranging from teenagers to old men, 1000 elves from Lothlorien led by Haldir (I couldn't find a canonical answer to how many elves are in the movie, let's just say it's 1000). And of course our boys Aragorn, Legolas and Gimli are there. Eowyn is with Helena in the caves. They are not allowed to flee.

Same prep time for the defenders as in The Two Towers, but they don't have any detailed knowledge about their enemies, e.g. they don't know who Achilles is. They know that Helena is the target of the attack and they will do everything to keep her from being captured.

The battle starts at dusk, and at the next dawn Eomer arrives with 3000 Rohirrim. The Greeks are unaware of any reinforcements coming until they spot them. Gandalf fights with them but he’s not allowed to use magic against the Greeks, only fight with sword and staff.

Win condition for the defenders: Kill all Greeks or break their morale and make them retreat.

So the defenders now face a massively bigger army (50k+ instead of 10k Uruk Hai) but the individual soldiers are weaker and worse equipped. How does this play out?

Round 1: The Greeks have battering rams and ladders.

Round 2: Additionally, the Greeks have one of Saruman’s explosives and they know how to use it.