You have a pretty neat little game engine here, but there are a few points which I think need some work.
The controls felt excellent... nice and tight. This made jumping and shooting feel very natural.
The main character has very nice walking animation.
I had one issue with enemy pathing... I don't like how the green enemies can "airwalk" if you shoot the red brick out from under them. I wasn't a big fan of the "superslide" that happens when you shoot them, but that is just personal preference.
A simple fix is need for the transitions from screen to screen. Right now a clone is visible if you jump across the screen. I found this unnerving as I kept thinking there was an enemy appearing below me.
I wasn't a fan of the music selection, but the big issue I had was that it was too loud compared to the game sounds.
Looks good and I hope you do more with the project.