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.
A tiny bit more polish would have done wonders for this game. I get that it's not meant to be a full experience, but it detracts a bit from the environment you are trying to create to see such things as enemies walking on air after you remove a block beneath them, enemies walking backwards, 2 iterations of the player when walking into a new room, etc.
small screen but fun!
I love your art style and the controls are good. I agree with the others, you should certainly make a full game out of this.
Well that was pretty awesome :). Loved every second!