The way the music tracks transition together smoothly is especially well-done.

I do want to mention a few bugs:
Sometimes, the opening life-drain animation sticks in-progress, and then life drain is just on all the time without having to press space. That was the case the first time I played and it made the game a lot easier. I think it might have something to do with framerate and the timing of the animation, like maybe the intro animation logic is waiting a certain amount of clock time instead of checking directly if the life drain's finished happening. (Also, without that bug I wouldn't have realized the animated trees are drainable, so a hint about that might be useful.)

If I kill the next-to-last boss and die at the same time, then when I respawn the door behind the boss is open but the boss also respawns. I went past it, and it ended up chasing me through the last boss all the way to the ending cliff. It killed me during the ending animation, and then when I respawned I was stuck in the ending animation. Possibly the code that spawns the boss should check if the door is already open and not spawn anything if it is.

I can also corroborate Misterm's report about the camera not switching back to the player after killing the bottom boss in the crossroads area. Possibly this happens specifically when killing that one first before the right-hand one.