I'm really just having a lot of trouble making any progress in this game at all.
Edit: Oh wait, there's double jump, I spent 10 minutes at the beginning of the game just jumping back and forth and throwing the ball around.
Edit2: Stuck again at the door. Artwork's really good though, and the mechanic's neat, though it does get a little frustrating on the moving platforms because by the time you jump they've changed position since when you could see them.