You should remove the donation prompt, it's a little off-putting for a game jam game.
I found it very unfair the zombies could come through walls at me; I could barely move, let alone solve the maze. And then it cost 100 cookies to get an upgrade? That's way too expensive for me, I died before even reaching 50 (after zombies pushed me into a wall rendering me immobile).
Maybe I just suck, but it seemed to difficult.
The shooting felt really nice and it was fun to kill zombies, so I reckon with some difficulty tweaks this would be really nice. I so wanted to try that SMG too...