A bunker consisting of no more than 20 brushes with a single texture and no discernable light source is not exactly high detail..
though performance should be good!

Okay, to build a nice bunker you may take a look at games like Castle Wolfenstein, Medal of Honor, Call of Duty... even Doom3 and Quake2-4 should give some nice impressions. Basic layout first, then detail... ramps only where they make sense.. metal stairs are fine, maybe some inventory like computers, tables, crates, anything... and something emitting light, maybe some neon lights?
Texturing all in a concrete & metal theme and you are done!
Cheers, Danny