Four levels, mostly small (well, except the last one). The first level is the only marathon level EVER to make me seasick. Be warned. Other levels: The Big Donut will make your eyes bleed, if you're not careful... lava everywhere.
Very simple map, with a large water column in the middle (it's where you start), an opening to the outside ring about halfway up, and a wall surrounding it that you can climb for bigger weapons.