I like the style of this game and designs for the little guy and the enemies! The music is very relaxing too.
I like that you implemented the ability to jump higher by holding space. However, I can't stress enough just how much this game needs coyote time. It became really frustrating falling to my doom over and over because of that.
As far as level design goes, it was fine, with the exception of the certain Dunes section and and Castle level. Dunes for some reason had some platforms that just didn't have collision and I'm not sure if it's a bug or an attempt at trolling, but if it's the latter, it just doesn't work for this game specifically. If you wanted fake platforms, they should at least have some kind of sign to tell them apart, or have certain platforms fall down after a few seconds to create some urgency and then reset them. The castle level really didn't need to be that long, since it felt more like a chore than fun platforming, and lacked general obstacles or enemies on the way up.
Despite that, it's still a nice little platformer!