Short, sweet, to the point. I really like the concept. I think it could be easily expanded. Big note, give us a metronome or a backbeat or something to sync up to the thing we're creating. There's not a good way to time your own beat. A simple 4 beat count in (maybe you could set your own tempo?) and a metronome would make this a lot more fun! And yeah, visual polish would be nice, although I really like Enemy Placeholder~!
Great work!