Looks incredible, Youme. Excellent work.

A few questions/suggestions...
Is the sign going to have something on it? Right now it looks out of place.
There's a black gap in your rocks, just above my crosshair. Not sure if it's a displacement issue or just the lighting, but I figured I'd mention it.
This is up the left side of the building, immediately past the cart on stage 2. You can even see a checkpoint if you jump. Obviously it's just a small chunk, so a single wall coming off the back of the building would work fine.
My only other suggestion is one I've posted before and I'm not sure if you're planning to do it or not. During stage 2 you can start walking out of the attacker's spawn back toward stage 1. Obviously you have a player clip there, but it still feels unfinished. A small fence going across each side or even a "No Trespassing" sign in the ground on either side would really complete it IMHO.