I loved the art, and the music never felt old.
I have only one thing I can complain about, and that is hitboxes.
Either spikes and lava has too big of a hitbox or the player does.
I could stand next to the spikes and lava to lose.
I did enjoy playing it, music and sound effects were great. And the feedback when hitting an enemy was really nice.
The boss was quite easy, you can stand outside his range and shoot him without any risk.
Good job!