Fun game! Kept me playing for a while. Some minor gripes:
The enemy path is predictable so after a while I figured out a strat where spamming throwing knifes when they float towards you practically takes down an enemy instantly.
The enemy hp would not always display correctly, something saying 0hp when they're still a balloon intact
Personally think the payout of the higher levels should've been a bit higher, because if not for the strat I used it would've taken me much longer to beat.
Other than that I think the upgrade system is well made and I enjoyed playing this. Well done!