The sfx and music fit the game well, would like to hear the buy menu open up, like a small rewarding Cha-ching to get you into that buyers mood lol. The upgrades feel a bit muted or slow and the enemies feel a bit to fast for the beginning, maybe adding slower enemies initially and then a small group of faster enemies later would help the player acclimate a bit better, once the player starts to get the hang of the mechanics. The mechanics are very quick to pick up but hard to master so that is a good thing. Ok that's it for my input hope it helps and it's a fun little game think it would sell if developed further. Cheers
Edit: Forgot to mention the graphics look good, a little more juice would be good but the basics are there.