Such a simple, yet fun concept! I like that the two types of zombies have different movement. The only thing is that in most maps, there's a spot where you can just stand and rack up points without risk. I think if you created more enemy spawners while proportionally decreasing the spawn rate, you could have a game where the player is constantly on his/her toes, trying to find out where the zombie is coming from.
The art style is amazing! It's kinda creepy, especially when the zombies start spawning. The first time, I was overwhelmed very quickly, which fits in with the zombie apocalypse theme. And of course, the art style is pretty retro. Love it!