This was a great and satisfying game! I managed to beat it after a while, and I really like the implementation of the theme. the graphics, everything!
However, for some reason, I accidentaly pressed ESC instead of X in the shop, and my progress got completely erased. This was pretty frustrating, so would be nice if it gave you a warning like "Are you sure you want to quit to the menu?"
Also the game is unnecessarily long, with a really plain ending, I would reduce the number of waves, and make the different enemies come sooner, it gets pretty stale after a while.
But great job!!