Shaped like a sledgehammer, with ammo at one head, the hill at the other, and SPNKRs at the bottom of the handle. Small enough that carnage should be extreme. Nice use of alien textures.
A large arena with four hills. The hills are extremely vulnerable, from many sides, but only accessible from one point (each). Nice flow, and pretty to look at, as well. (You'll have to hunt for the teleporters the first time through...)
Horrendously annoying water level (although that seems to be the point...) Four areas, separated by ledges... primary mode of transport is the water, which goes up and down a lot (and never when you want it to...).
Everyone starts out in a small, rectangular room filled with TOZTers and 3x canisters. You teleport out from here to a small cylinder, with shelves along the sides... a teleporter at the bottom brings you back to the top. Simple in layout, more weapons than you'll ever need, brightly lit. Expect really, really high carnage rates.