Thank you very much for your feedback, Akien.
Yes, it's pretty easy. I made the mistake to test only in window mode, which makes the game a little harder. Also I probably should have made more clear how the game works:
On each wave you have to kill 1 more zombie then in the wave before. You have to kill 1 zombie on wave 1, 2 on wave 2, etc. Starting with wave 10 you have to kill all 10 zombies. This was originally a temporary solution and I planned to figure out something better but I ran out of time.