Cool game, overall I enjoyed it.
Graphics and audio while simple, have some charm to it. It's easy to make 8-bit sounds jarring and/or annoying, so I'm glad that's not the case here.
Forcing you to pick a type of gun every so often puts an interesting spin on the idea of picking up upgrades, as you may find yourself in a situation, where you previous upgrades are not optimal for the new weapon.
Flying enemies are im my opinion too small, where the jumping slimes are pushovers. Albo combination of low accuracy weapon + flying enemies + bullets going through walls is almost always a death sentence.
Game is very hard, but I managed to get to level 15. On which level is the boss?