Removing the platforms after a certain period of time is a great idea. It would be nice if they came back after some time has passed, but the level is still passable after that. When squashing the enemy with a box, it would be nice actually see him get smashed :). And maybe the boss arena could be larger, I died many times by falling down. Also probably a bug, but when I died in the boss arena, my hearts were kept at the amount in which I entered the scene, which was 1/5, so maybe fill them back to full. Other than that, it's a really nice platformer, the visible plants and spikes in dark level are great, and the key picking mechanic as well.