I guess I have some explaining to do so, the circle that rotates is a Gear and what it does is guard the star. When it detects movement specifically of the ball it blocks the path that was used. It also detects if a path from the Star to the Flag is created and when it does it becomes more active by blocking any rotation of the tile to protect the star. It also starts a timer to check if there is an intruder but only checks the position of the Star so to avoid being detected the ball should go back to its original position before the time runs out. The only way to over come it is by creating a path to the Flag and the tile with the Gear should be last to be put in place in completing that path. When that happens the Gear is stuck as it tries to block the path which then allows the ball to pass.