About checkpoints: I know it's just a demo but in the full game I think it's a good idea to have a checkpoint after each new
major challenge. It's easy for you because you've been programming and testing it constantly but for a new player you spend ages trying to get through the bombs and then you encounter the boulders. Something you haven't seen before: do you jump over them? on them? freeze them with charge-up? Oops, too late you're dead. Back to the bombs. You know?
In later levels, when the player is used to the controls and the gameplay, you can string challenges together without checkpoints to make it more difficult.
It's a fun game. I'm close to killing that gorilla.