i really liked the demo! felt like something you'd play at an arcade cabinet. a few suggestions:
-the cursor movement felt a little slow to me. although im sure you plan to put a mouse sensitivity slider in the final game
-it was kinda difficult to gage how badly i was hurt? it took me a while to find the healthbar in the top left corner
-those guys that shoot--it seemed like they took more than one shot to kill, but i never really got a good indication of whether i hit them or not. with the knights it was no big deal because i mostly just stood in one spot and unloaded on em, but with the shooty guys i had to do a lot of ducking and dodging to avoid their bullets and so it was hard to see if my shot actually hit em.
-the hitbox on those powerup things is a little too small i think, i kept walking over it to try and get the little popup text to appear
overall though, really solid game so far!