Feels good to play! I definitely agree that the zombie spawns feel unfair, especially when I'm near the edges of the map. I was getting overwhelmed with zombies a lot as well, so maybe some down time between waves would make it feel a little more fair. I did like that it was difficult, but I feel it could be better if the dials were turned back a bit, as I pretty much never risked going to another floor since I got overwhelmed so much and lost so much health.
Other than the balancing, the game is really well polished and feels really good, so great job!