Fun little game you have there! Its quite innovative in a way that it is unusual for tower defense turrets to despawn after a while. I just wish i knew when that happens. I found myself losing, by my freeze turrents despawning, just when the fast enemies rushed in. A little bar indicating the time on each ball would have been nice. And also a warning for when the fast ones come. Graphics are simple, but have a certain style to them and work good in combination, nice job! Sounds and music are okay-ish, a little too generic in my opinion.
I personally found myself interested in "Game Feel" lately, maybe you want to read up on that. Bigger bullets, more explosions and particles, hit effects, screen shake, etc. Those little things do go a long way for the game to feel satisfying.