...i really like this map ! When i just stand somewhere in this map and look up, it looks so great !
I agree though that is still is very dark, but that's how it was intended to be, i guess. This map has a very dark atmosphere, and making it "too" bright might not work too well.
I also agree about the textures: I use these textures a lot myself and i know how hard it is in this map to find the right details...these textures are very "squared", so i always end up with square shapes, square rooms etc which work fine in sci-fi maps and "tech-like" stuff but not necessarily with all other kinds of maps. Maybe a bit more texture variation would help, though i'm not sure what could be added with this textureset to make it look better.
Personally i think, using some different textures to make it look more "medieval" or "aged" would work perfectly, you still could use these metal textures then for each metal-looking stuff ?
The skybox plus the cloud layer just is an awesome combination...congrats about that :)
All in all i can say, it's a map with awesome atmosphere which would be worth 5/5 if there were some more details or some other, additional textures that fit in the scenery.
4/5 :D
EDIT: Lol, forgot about the screenshot: This one is WAY too dark. Not that important though, but still ;)