This is a really cool concept! Being able to maneuver the skull around using the screen's boundaries is a very interesting mechanic, and I had a lot of fun playing around with it. I liked the incremental addition of new obstacles/challenges, especially challenges attached to the screen's boundaries. I noticed that some levels had a considerable spike in difficulty, but I personally enjoyed that. The amount of levels available is great!
In case you guys plan to continue development on this game in the future: While playing around with the movement, I noticed that under certain conditions, you're able to glitch in and out of some of the map's boundaries, which potentially allows the player to skip all of the obstacles. You can recreate this bug by moving the skull close to a tile, and quickly bumping the side of the screen against the skull. If your game uses a velocity system for movement and bounding boxes for collisions, this might be because it is possible for the skull's velocity to be greater than the size of a tile, which will essentially skip the tile entirely during a position update, thus also skipping collision checks. In the past, I was able to fix this issue by capping the max velocity at a value less than the size of a tile.
I had a lot of fun playing this game!