As it is a demo, it's pretty good, HOWEVER, it was quite annoying when you died. There was no pop up to restart the game. Only after I beat the boss, did I receive the instruction to 'press backspace to restart'- I kept reloading my game because once you died, there was nothing on screen except the retro music and the dead player.
One more thing- its a simple concept but I reckon the response time to hit an enemy and to move up and down was slightly slow, not as responsive. For example, hit enemy, try to dodge an object, pressing up repeatedly, does not respond instinctively. Otherwise, good game. Looking forward to full game!