cool game! I like the game mechanic. The first level was the most fun, because it felt like I had to actually AIM the ball. The last level was "easy", because I just kept the ball in the middle, and wiggled it around.
Perhaps, given more time, you could add more mechanics -- for example, less enemies (to promote aiming, vice wiggling), and maybe different types of enemies. For example, maybe an enemy that orbits around the center, shooting bullets in. You can block the bullets, but you need to hit the orbiting enemy to stop the bullets. That would require aiming.
Just thoughts, well done!
Please review mine, if you have the chance.