Really liked the idea behind the game! The music was nice, but I don't know if it fit the vibe of "I'm corrupting people to build a zombie army"? I really liked the idea of upgrading my zombies to take on new challenges, but I disliked that I had to just play the same level and keep losing to grind up the hearts to do so. Even smaller levels that maybe gave a bonus when you finished them would have been nicer to me. I thought the AI was decent, but I think the next level of polish would be tying it all together. Right now the zombies sometimes swing and run through their targets, they miss, it feels very... 'loose'? If that's the right word? I think it would really come together if the animation's were better paired with the movements and actions.
I also think the upgrades were great, but there wasn't really a choice in any of them. It might be more interesting to have Speed OR Strength, where one locks out the other etc. It leads to tough and interesting player decisions. All in all though, I think it nailed the theme, I enjoyed it and had fun, and congratulations on getting a game finished and submitted!