This was easily the most fun game I've played so far. I got so invested in the tower defense and upgrading felt rewarding. I might be biased just because I find tower defense games super fun. I didn't understand how the upgrade system worked at first, so maybe an indicator on the button when you're able to upgrade might guide players more. Another thing is that I wish you could delete/sell towers for a less than what they cost, because sometimes I would accidentally place down a tower somewhere and it would feel unforgiving to have it there forever. As a whole though, oh my gosh this was fun. I really hope you expand it!