Interesting mechanics! This was very fun to play!

Pretty fun concept, I feel like there was a lot of trial-and-error involved (rather than strategically thinking) though.

There's some room left to polish things up, there's some small UX gripes like "start wave" button being darker than every other button (made me think I needed to do something more before it became active even though it always was, for instance) and the way the music doesn't loop properly is a bit grating.

The upgrade mechanic is definitely what makes this game work. It was fun figuring out how to get towers to get the precise right number of kills to upgrade them without wasting any kills. Nice work!

Good concept and good execution! I particularly liked how leveling up towers works. It would be nice if players could see what the future enemy waves and debts will be for the rest of the level, so that players could strategize without playing through the level first.