Essentially this is a pyro sumo ring. Your health recharges constantly as you play, so it takes a crit (2 simultaneous crits in the case of a heavy) to kill you. It's often easier to use your airblast/FAN to knock people off the edge than it is to kill them with bullet, which they can and will outsmart.
The .zip file contains 11 maps, all various versions of pyrumo. The latest (and the one you should play) is version 4g. I included the other ones so you can see just how buggy pyrumo is/was.
The entire top area is one big control point.