The controls are awkward. Make it so it can use arrow keys or aswf, and make it so you can use up to jump when you're not on a ladder.

Also, it's too hard to tell the checkpoint flags from the end of the level. Normally that wouldn't be bad, but you also made it so that until you get all the diamonds, the end of the level is functionally instant death, forcing you to start over from the beginning and gather all the diamonds. I recommend making it so you have to press some interaction key to finish if you don't have all the diamonds and coins.

I'd also recommend making it so the diamonds stay collected on repeated playthroughs. Maybe the coins too.