Dude, that's really nice work.
I'm gonna post some criticism, not because I think your work is bad (it's excellent) but because you said it was practice..
Basically the join between wall and floor, and floor and ceiling is too harsh. Perhaps you could use a tiny bit of the floor segment at the bottom of the wall, so that it appears more seamless
(use a new layer with the floor segment, then errase some of it with an eratic brush?)
But keep up the good work man, it's certainly gonna make your levels more impressive.
butters
"It's often better to keep your mouth shut and let people think you're an idiot, than to open it and confirm their suspisions" - Noel Coward