With this game being a short and sweet proof of concept, I think it offers some fun! It has all the pieces to be a good game, with some good sprite work,  engaging gameplay, relatively simple controls, and a fun atmosphere. The 5 shorter stages have some good ideas and showcases of the gravity gimmick. There are a handful of enemies that also take advantage of the gravity system, which makes for some really fun encounters. The shifting of boxes to gain additional points for your score is both satisfying and further ads depth to how you approach enemies. 

I think the "difficult" mode could use more than just higher Hp enemies and the player taking more damage. What I think would truly make it was if the enemies had more variants. Maybe some new sprites, or heavily modified enemy sprites that have new attributes, like say the turret enemies fire 3 shots rapid fire, the balloon enemies could explode an AOE when being defeated, the yellow claw guys projectiles could leave small shockwaves on the ground. HELL, the boss could move his canons slightly up and down, requiring the player to crouch on to avoid. Smaller things like this would make a "New game+" so awesome

Only real critique is the music wasn't something I was huge on. The Boss theme was actually dope, but the stage music wasn't my cup of tea. But aside from that I really enjoyed what I played, especially for a demo. This game is pretty polished while only meeting the surface of its potential.