I really enjoyed playing it, it's really well done but I think is a little too difficult. The prices of the upgrades are a bit too high for me, especially when you get to the round of the boss, if you have already bought some healing previously, healing again costs decidedly too much. I would also reduce the probability of slimes spawning for at least the first 2/3 rounds, because since they have a lot of health, in the initial phases are very difficult to defeat.
Anyway really nice work! with some improvements I think it could become a really fun game.