Great game! Quite challenging (in a good way) and kept me hooked. Simple, sleek, easy to figure out but hard to master. Nothing major to change imo!
Improvements wise, maybe some background music, and a menu? Turret upgrades would be awesome too but will complicate the game a lot more.
I got to level 4 so don't know if the future levels implement this but a different background or something could add a nice change but that's just opinion :)
Can see this being mobile friendly too! Maybe a few more hints may help when the player fails a level.
Overall, excellent work! I have lots of lessons to learn :)