Controls (listed ingame too):
Right & Left arrows to move.
Up & Down arrows to climb the pile.
D to jump. S to shoot. A to switch ammo type.
M to mute all sounds. Q to cycle anti-aliasing qualities.

A few hints (you might want not to read these):
>> Notice you should climb higher yourself in order to make the enemies go higher too, because they go after you, in all directions. If you stay on the ground the pile won't grow higher than a certain height.
>> The flamethrower is very effective: Once you light someone up, he will run around and climb the pile higher and higher, infecting other enemies with the fire as well.
>> The big lazer gun affects not only the enemies: It will also suck normal ammo and health packs, as well as lazer ammo. It also affects you - try jumping in different timings when using this weapon.

In this game you get a small arsenal of 4 essentially-different weapons. You use it to kill the enemies who flow to the arena. The more you kill - the more will come, and the higher will your pile arise.