This game is super addicting, with a great gameplay loop, an adorable artstyle, and great implementation of the limitation!
The difficulty kind of stays the same around Wave 15, and I never could tell if a wave was starting or ending, other than enemies spawning (although, that could just be my own idiocy, lmao).
Add a few more enemies, increase difficulty curve, and I'd say it would be pretty much perfect! Great work, guys!