Enjoyed the monochrome pixel art. I also liked how each level had unique mechanics and that the last level combined them all together. Very creative and I liked that they all made different uses of the "Countdown" theme. The player character controls well.
Some of the sound effects seem louder than the others, particularly the little sphere item that knocks you upwards. The spikes also looked too similar to the terrain in some places, so I found myself jumping into spikes not realizing they were spikes at first. I think it would help to make the spikes more visually distinct or maybe even a different color.
Also, for some reason, the dive move wouldn't work on my gamepad. The K key worked though. Not sure if its my Logitech gamepad or an oversight.
Overall, nice job on this game!