Maybe it is just me, but I had a bit of trouble on the left wall, since the movement keys are "swapped" i.e. when I press "left" the character moves right (relatively speaking). The right wall and the ceiling made sense.
I think you could also play a bit with the gravity of the player character. It accelerates slowly when falling whereas moving left/right is instantaneous. So maybe make the character react a bit faster to the change of gravity?
All these things don't really matter in the first few "puzzle"-levels but they become more important when you are running away from enemies. The enemies also feel much more mobile than the player, maybe they could wait a bit before they change direction?