Glad to know it works! First of all, thanks for the feedback.
About the walls/unseen thing, yep, I thought it could be better to let them to be the same color, but I'll take that in mind.
About the navigation thing, zombies just know about a few hundreds of pixels around and player position, so they're a bit blind. I just took this jam to implement boids, map generation was not planned but definitely a good feature in the end. It would have been good to code a higher level pathfinding thing with an A* or something for each group of zombies, but I had so little time. I was on my own for everything (code, graphics and music) but for the main screen (a friend did it). Also, I had a little more than half a day consumed by work, so I couldn't spend as much time as I wanted into this.
About the footprints and bloodstains, yay! it would definitely be a good enhancement. I'll take it if I decide to rework the game a little.
Again, thank you very much for the feedback!