Nice idea, meh execution
The good:
The concept (although a little more backstory would be nice even as simple as a starting "where am I?" cutscene)
challenging
The bad:
Some of the *avoids spoilers* boss attacks are extremely cheap, but this is mainly because of my next point.
No invincibility frames when being hit! This means you get situations where you get hit multiple time in a row and there's nothing you can do about it. I noticed this doesn't happen while you're in the air however only once you've landed back on a platform. I know you're encouraging the player to aim for a perfect run but this could still do with tweaking, maybe even a feature you could turn on or off.
needs more speed between hard sections