Nice gameplay! Really fun, the early game is a bit slow and the rate of earning currency is quite balanced .. but at mid game everything is shooting (and dying!).
As a small improvement .. I would have added a "Send next wave" button so you can stack waves if you are way ahead. Also I would love to see a more "cursed" environment, the bundle where you took the assets contained a few other visual elements that would definitely help improving that.
General this game is Awesome, really like the idea and would like to see an iteration on this!