First of all having a house in the corner of the map is a terrible idea, fill up the map and put the spawn in a reasonably decent place, having it in the corner makes it look, meh.
Second, I'd like to add that a square arena doesn't cut it and the map is way too small, it's also incredibly empty and why isn't there any baseplates for the ground?
The walls of the arena should blend in and provide somewhat of a natural barrier from getting out, I'd also prefer the zombies spawn in hidden areas like a tunnel so they come out, maybe even holes in ceilings of buildings, the possibilities are endless here, I don't think you put enough time into this build, if you work on it enough, it will look much, much better.
You should add more ways to get around the map to keep the gameplay fresh, new areas, new ways to get to those new areas, It's very empty.
