Well I'm working with much more modern Engines such as Torque 3D, UDK and Unity where they are able to use Meshes as collision instead of BSP based modeling. Most things these days are done on the fly such as lighting, BSP used to used a lot of prebaked lighting and collision. My concept art was very very basic, most of the time we properly plan out what is what and how high places are but because I was doing it myself I just mentally knew what I wanted in what areas.
I used to do quite a bit of BSP stuff way back using Torque Constructor/QuArk/Hammer and I always did my levels being able to approach from all angles, where in my videos my canyon area is the only area you'll ever be in so the outside is just empty to save polygons. I mainly did this to show off my ability to knowingly remove unnecessary polygons from my game scenes but yet keep enough in to show details from all angles.
Overall your structures are very building like, i.e. 1 way entrance to a room where in game design 1 door = Deadend - death trap, 2 doors = a hallway and 3 doorways gives the player decisions to where they want to go.
