i feel like there should be some sort of score counter to add replayability to the game.  at the very least, a timer to show how long the player lasted could improve the game a lot. 

also, from what i saw, none of the zombies have any differences besides health. i encountered the normal zombie, the helmet zombie, and the mutant zombie. you could change the zombie's speed, or the amount of damage it does to the house. maybe you could make some zombies target the player instead of the house. all of those things are pretty simple to do and yet add a lot more variation and make the game a little more interesting

all that being said, this is still a very impressive game, and still ends up being fairly fun. good job!

simple, but the difficulty increases too fast

Good game, but the difficulty doesn't seem to come from complexity itself, but from the slow and clunky mechanics. It improves with the acquisition of the rifle, but it doesn't offer the possibility to get upgrades or something similar to make defense easier... Still, good game.