Bobs, bobs, and more bobs. Some are explodabobs, some are not... Play it long enough, and you'll be fighting all of them, like it or not. A good test of how long you can stay alive against bobs. Update tweaks weapon availability.
Lots and lots of twisty passageways, leading into monster-filled rooms. I'd recommend turning aliens off for netplay... or you're going to have a hard time finding your human opponents.
Concentric hallways spiraling out... hence the name. There are shortcuts at some of the outside corners, and at least one secret room. Also comes in a solo version.
A small map. More than two or three would get really crowded. A couple of secrets, but basically, this is a first effort... sort of a test bed. (There's a little of everything...)
A small arena with some not-so-apparent ledges. Watch the nasty platform in the lava.