Nice work. Seemed to get a bit slow or 'clunky' at times and the bullets sometimes went through walls and hit zombies sometimes not. (WebGL version).
Maybe run an optimization pass over the game e.g. is it pooling and re-used resources e.g. zombies/tiles or updating some things too much e.g. UI elements every frame.
Would have loved to be able to get into and drive the cars.
Gun seemed a bit too accurate for a starting weapon, but great potential to build up on the work you've done.
Maybe it could be more dramatic with a night time and torch light or foggy weather?