Really enjoyed the game. I have a few tips but I think some have already been mentioned:
An animation when the zombies spawn so it looks like they come from under ground
Maybe add a turret behaviour to the hero sprite and make him auto aim at the closest zombie. (this will also make your game more mobile friendly)
Start with one room, when "zombieKills" reach X amount, reveal the second room (on the same layer) etc. When all rooms are revealed, spawns a boss and call it a level
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