Big. Really big. Lots of hallways, some dark, some not... a few large rooms to blast people in. (Most of the hallways are big enough for blasting, so they're not completely necessary, but...) Doors and elevators are quite slow, so don't count on a fast escape if you run out of ammo...
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As the title implies, five levels. These are generally large, wide open, rambling levels, mostly well-lit. Aliens abound, and weapons range from plentiful to meager. A cool trick I don't think I've seen before was used on Doors of your Mind.