Cool idea, works smoothly but it starts and you are already playing. I'd suggest adding a tutorial screen even if it's just a controls layout, other than that the game feels smooth to play and is well put together. If you could maybe modify the sound effects from the zombies so they are positional audio, it would help with the game feel a lot and it'd not be as hard to know where they are coming from. And maybe some background music, too, though I'd set the volume VERY low for this kind of game so that it still allows to hear when enemies are nearby.
Cheers :)