This is a shooting "game" (if it qualifies to be one). Your objective is to step on enemy's base territory (that green stuff providing you haven't changed colors). But since this territory is occupied by the enemy's defense players, you have to kill them first.
Like the idea? Don't click New Game then, seriously, because you think it's way better than it is, I swear. If you choose to click it anyway, then:
Use mouse to aim&shoot, arrow keys to move, and p key to pause.
Don't forget this is a zombie release - don't expect much... after all, something more sophisticated was planned, but this was done, then this project was scrapped :) I guess it's because 90% of time spent on this game was spent on creating AI players PS. The code is horribly horrible, I tried to comment some parts but...