Really cool demo, got everything except the big bone at the top of the building that has the two signs. I missed the red button the first time I entered the tower and collected all the bones around the upper level, you should make it stand out a little bit more, maybe with a faint glowing pulse, scale it up or perhaps it's just skill issue on my part (reminder that gamers never look up).
Controls were nice, I feel like I'm holding shift all the time, perhaps you could invert walking and running? (maybe making it a toggle within the options menu), I'm partial about the skid or slide the character makes when changing directions, it can be easily bypassed if you let go of the movement key you're holding before moving on the opposite direction, so it's not a big deal, perhaps you plan on adding a side flip like in Mario 64.
Another small nitpick: I think the shrinking of the character on every jump is a bit too much
I don't know if it's my GPU(GTX 970) but I had this graphical bug with the portal at the start of the game.
Overall it's a really fun platformer, the texture work is phenomenal, I really liked the bluish hues of the cave passages, the art style is on point, you will be getting that a lot. Good luck with development!!