Great game overall !
First of all, I liked the soundtrack (literally stuck in my head for an hour after playing) and the visuals. It looks good and feels good but it isn't without flaws.
I think you could easily add more visual feedback. Here is a few things that you could add to improve it: - Health bar on top (or around, or anywhere else...) on the enemies. - A written level (or different kind of indication [maybe change of sprite??]) over the towers so we know what level each are after upgrading them.
Also, I will not list the bugs because other people did but it wasn't too bad for me. Try also balancing the overall gameplay (maybe amount of currency earn after each kill can increase depending on the wave??) and of course, limit a maximum amount of turrets :smile:
Otherwise, it was a fun mini-game. I had fun playing it and hope you had fun making it!
One last thing: for this kind of mini-game (that doesn't require too much performance), try exporting it to WebGL (HTML 5) so there isn't any download necessary. It is always faster and I know many people tends to avoid downloading every game. :smiley: