There's only three problems with this level: the background, the length, and missing graphics. When you change backgrounds, use Paint.NET to edit the image so that you don't get all the fuzzy artifacts like you have for your background. The level seemed to drag on with nothing new to see, and it kind of over stayed its welcome. I really did enjoy the gimmick you used, but you used it a bit to excessively and didn't really implement it in any different ways other than level navigation. Also at your midway point, there's missing custom graphics so it just looks messed up for me:
You should try and do what Nintendo does and start the level off by introducing the player to the gimmick in a safe area, and then slowly ramp up the difficulty and then have the player approach the gimmick in a new way to advance or uncover secrets. It's up to you to come up with those ideas though. Try playing some Mario levels and pay close attention to how they fit that design scheme and try it for yourself. The enemy placement was overall done well but sometimes they look a bit silly like right here where an enemy is intended to be flying but moves along the ground:
Really if you just fixed the background, knocked off a section or two, and then tweaked the things I mentioned above then you would have a much better level than what you have now.