Very creative, even with such a simple mechanic there's a fair amount of complexity. I'm glad the enemy movement patterns start including ones that move multiple spaces because in the early levels you can spam the back button to always take out every enemy. That doesn't remain the case after a few rounds, which makes it kinda an exploit you can only make use of in the early game and then you're forced to abandon it, which is neat.
There's also so much polish! All the smooth gliding UI elements and the enemy animations and extra background details, it's all great. And the music's perfect, the clock ticking being incorporated into the music sounds so good.