Some stuff I noticed with the destroyed Citadel in episode 2...
Was doing my own research on the Citadel ruins, and noticed the textures they used for the ruins are different for the two levels we see it (Beginning & Hunter ambush).
The first is several flat materials (textures) layered within the skybox for the illusion of depth (All the pointed out bits). As shown, the primary portion in front of the Superportal Vortex (Main 1) is facing left, but the next time we see the Citadel, that part if facing right instead (which, fascinating detail, is one single material since Valve didn't have to worry about the proximity). My best assumption is what you see on the right (That or the power of the Superportal dragged the portion the other way), but it's a curious detail, though I'm uncertain of where the we are positionally between the intro and the ambush.
Speaking of position, another thing I discovered was that Valve gave the illusion of distance for the second time we see the Citadel. When I looked at the skybox, I noticed that the vortex - both the beam and cloud - was the same size as the beginning. It's only the skybox materials that changed to make it look further away. A neat detail I noticed and you gotta commend Valve for.