Nice art. Music fits the tone fairly well. The premise is pretty original, and the 3-barrel firing is a very nice touch.
I'm not sure why this was happening, but I tended to drift toward the top right corner. Unable to maneuver out of it, I would eventually die in that position. Unclear what was going on there, whether it had something to do with the game or perhaps my own mouse.
One suggestion is that the zombies not spawn without warning in the middle of the court. They can either spawn outside the visible boundaries, or in the middle of the court with some warning in advance.