Very original concept. Aesthetics are cool as well.
I think perhaps the idea of being a circle or going the easy way of becoming a square could be explained or emphasized a bit more clearly - I completed the game once as a square, I didn't really understand the point and only got it after looking at the comments here. And completed it as a circle afterwards. The idea itself is great though!
Maybe the game could also use slightly more variety in the platforming challenges. I understand the square ones being trivial, but the circle ones were almost exclusively jumping on sequences of 1-block platforms.
Oh, and the camera has too much lookahead, which is a bit annoying when jumping left and right between 1-block platforms - your view constantly makes those huge jumps back and forth, often not actually showing you the block you want to jump on next.