A decent game, even if a bit frustrating in later levels. What really bothers me is that you made a game about blowing zombies into tiny pieces of meat with cute innocent soundtrack and graphics. In my opinion, this totally misses the point of making a zombie-themed game.
Funny little detail: the soldier's helmet with the white stripe looks like the headgear worn by German Freikorps soldiers back in the 1920's. Yes, I do know its a visual pun on the iconic Vietnam-era headgear, but some German players may feel a little discomfort with this ;-)