You got an interesting ideia there!
The pixel art style really looked cool, even tho i am kinda bias because i love it, i think u did a great job there!
You should try to work on some points to make it better, like:
- Make a tiny animation for zombies when they attack so the player gets more visual feedback then the numbers and a quick color change
- Create a more gory scenary, so it feels more impactfull
- Create different type of humans
- Since you are doing pixel art, and you have loads of stuff there, you should try to work on diff types of line art with the sprites, since the more important are the floors, maybe they should be more saturated and more present, and the others more faded and with a slimer line
Hope i can help you with this opinion and keep the good work fellow gamer