It was actually way more fun than I thought it would be! I actually made a game similar to this with stickmen instead!
i think that since you made the player and enemy art using simple squares, you should have went for simple squares for the floor and platforms as well, they look out of place
Particles are cool but i think it needed more "game juice" like screenshake when explosion, firing, and enemy death. Some particles for the enemy death instead of instadeleting the enemy would have done better as well
Overall great job!