Playing this made me deeply nostalgic for playing zombie defense flash games in the 2000s. I would have happily played this for an hour.
Your gunshots, explosions, and splatters were all satisfying to see, hear and feel. The pixel art was nice and cohesive.
Seeing the zombies destroy the wall and then climb on top was a huge surprise the first time that it happened, it is was a genuinely scary and cool moment.
I do wish the game was a little faster. I would personally prefer faster zombies that die in fewer hits with more drastic upgrades - but that's simply my taste. What you have here is nails the atmosphere of a slowly approaching horde.
Really fantastic entry!