This game is off to a good start, artwise. I feel that this spritework is very nice and polished, and extra details such as animation or if supported by clickteam, squash and stretch can add a lot of polish. When the monsters die, think of adding some small details like adding particle explosions, and having those particles stick to the surfaces for 10 - 15 seconds for extra. Screen shake can also be considered, and for 8 bit or 16 bit sfx, you can look towards chiptone for those type of sounds.
Please check this video out by blackthornprod for an example of what I am trying to describe.