It seems like the entire level is built inside a box. That is not the way to go. What you'd want to do is that you need to make interesting areas, and glue them together, making an all-around more interesting and more dynamic enviroment.
My first maps were also built in an oblong concrete wall, with buildings populated inside.
Try not to do so.
Besides that, I'd recommend better light_enviroment. Perhaps copy the properties of another light from another map? Or you can simply change the values yourself to more interesting angle of light, and add a hint of a colour, preferebly a shade you'd see in sunshine (Ex. Yellow)
I like the gullywash-style control point.