I played the demo, too. Good work overall! It feels very retro

Unfortunately this also applies to the handling, which is very 1993. Details I noticed:
- Alt+F4 doesn't quit.
- Keys are not rebindable. On a german keyboard, Z is far away from X.
- Menu handling: There should be a message that Z is "back"
- Menu handling: It's not nice to the eyes that selected options get additional characters to highlight them - please use color our slight scaling to hightlight.
- If you already swing the camera left and right, you might also add a little look-around with up/down
Apart from that, this game could be the next CaveStory - I enjoyed it. But please do not make the same mistakes as CaveStory - weirdly placed save points that need typing instead of saving automatically, and difficulty adjustments by simply not providing save points in front of boss battles and such... all power to the retro and everything, but there are things which are history for a reason.