Extremely polished game with a very simple, yet fun mechanic! The ONLY SINGLE thing that bothered me was the long cooldown between shots (maybe it was intentional) and the speed of the bullet projectile. Not that it makes the game harder or frustrating or anything, but I think it would greatly improve “juice” if you were to make a higher fire rate and a faster travelling bullet. To balance things, you can increase the enemies’ HPs!
Congratulations!