Pretty good demo!
There were a lot of nice little touches that I liked: The transparency effect for when your character goes into a room and the cracks on the screen when your character gets hit. I also liked how there was a shop between levels.
As far as improvements go, I noticed that the enemies quite frequently would get snagged on the corners of buildings. Also, the player weapon mechanics could use some adjustment -- I noticed that the player "bullet" doesn't always spawn from a consistent point. Would recommend making the shot origin dead center relative to the player.
I'd say you have a pretty good foundation to build on for the next version.
My own top-down shooter is a bit different in style from yours, but if you'd like to give it a try I'll leave a link below.