I really love the pixel art, the enemies in particular are fantastic and animated really well - the death particles are a nice touch. I did have any issue with the punching where it seemed a light tap on the key wouldn't register, you had to hold is down for a tiny bit to punch. Once I found I could hold down the key to punch continuously the game kind of stopped posing a challenge.
I had fun with this, a with a few polishes here and there (score/end goal etc) this could be really engaging, nice one!