If you have never heard of Smash TV,
read this.This arena sort of resembles smash tv, in the sense that its a box, you play from a GTA style top-down view, and the enemies fly out of the walls. Anyway, here are some screenshots. It was made using the zombie mod. It used fairly normal player options, except players were forced to use the top-down view model, the gun was locked into a horizontal firing position, similar to doom, and the weapon spawns took insane amounts of time (to simulate the random power-up drops of Smash TV).




And the entire arena it self, complete with stands and a glass floor for viewing.
