Hi i finished the first 2 levels of the game (i ended at the checkpoint next to a deep hole) and i think this demo is really excellent, its really hard and punitive but you keep getting better after each death and it feels great to come at the next checkpoint, the atmosphere is on point too.
However i think you should add an easier and less punitive level before the 2 first because the game starts really hard and i had to force myself to play it at first, and secondly i think you should really change the player jump sound effect (maybe remove his voice and place like a jump sound effect) and the soud effect of the spike when it dies is really annoying too
beside from that, great demo!