It's a fun little game with simple but nice graphics. It has it's flaws though - lack of sounds, no "return to menu" option (or at least I'm too blind to find it), balance problems, some levels have counter how many mobs passed and others don't, no ending - like really, you just end up with wave no 21 and nothing happens. You could add at least simple screen with how many mobs passed, some ratings (eg. 3 stars for no enemies passed, 2 stars for max. 10 etc.) and some back to menu option. I had some problems with placing turrets as well - sometimes I had to click multiple times in one place to place it.
Anyway I kind of like it and hope it'll develop.