This was pretty fun, and the art works well. Definitely could use some additional mechanics. Zhong317 had some good suggestions in another comment. I'd also love to see some sort of progression mechanic; maybe you can buy/find weapon upgrades, traps, grenades? Also my tactic was always to strafe around the perimeter shooting inwards, and that seems to work well. (At least until I ran into some sort of invisible wall on level 4 (top left I think?) Maybe some sort of mechanic that interrupts that tactic so players have to change it up a bit. Cool game though, nice work!