Looks good so far.
A few things:
I would scale down the textures on some of those walls, it's especially noticible in the first and second pictures, and it would look a lot nicer if you do this imo.
More details, as others have said, things that apply to prisons, but also random greeble, little brackets on the edges of stairs, small pipes, whatever.
The lighting could use some work too. There is some bleeding as someone else mentioned, and I would also advise toning down the ambient light a lot. Besides the fact that I'm personally disclined towards bright maps in tremulous, a prison should have a nice kind of spooky atmosphere, and being darker would help. (You don't need to have it be really dark, but try having the lighting coming from multiple weak sources, I think that would look nice.)
You also have a distinct lack of clipping which would really make it look better, and ties in with the greeble part. Try, for example clipping the outmost edges of those columns in the corner of the H and A bases, at a 45 degree angle, so instead of:
| |
|__ , you have: \__ (kind of, my crappy ASCII isn't very helpful maybe, but I hope you understand.) That same 45 degree clipping on the edges of the stairs and arches in the hall would look very nice too imo.
Hope you find this helpful.
