Great game concept! The art style is quite simple but very cute ^_^ The upgrade simple follows the same simple, but well thought-out pattern :^)
My only complaint is that the game is a bit too difficult. I didn't play it very long, but my problem is that the you get too little money from the slimes you kill and the upgrades cost too much. At one point there's just so many enemies on the screen that I feel overwhelmed and can't do much against those massive hordes with so little money.
Other than that, I really like the game so far! It'd be cool if you develop it more and maybe even add some medals to it ;^)