I'll just say I agree with the previous poster about everything positive. The aesthetic is spot on, and once you get a bit more movement under your belt, trying to navigate around obstacles and flying heads is a fun puzzle.
That puzzle seems like it's the core of the game, and I feel like you should lead with that, maybe give the player that first upgrade right off the bat, since getting enough gems for it seems like a bit of a die roll.
I'm actually pretty happy with just the single enemy type. It lets me just kind of zone out and map out optimal routes without having to juggle a bunch of different enemy movement patterns.
The audio could use some work, but I'm sure you know that.
All in all I'm quite happy with this as-is, and I can't wait to see what you do with it in the future!