I played for a while--I found the door, and then I found the key, and then I finally died, unable to find the door again. :(
I find it fascinating how easily I was able to see many of the same notes over and over, while the door room eluded me... the maze was really quite intricate, despite being rather small.
I'm not sure I understand what exactly the heartbeat mechanic is--I lasted for I believe at least ten or twenty minutes without dying, and I never pushed shift or space. I definitely had enough time to beat the game had I recalled the path to the door.
But I do like how detailed the maze is. It's also a small thing, but I kind of like that the player's rotation is separate from their movement direction. It might seem kind of weird to have to rotate to read notes, but it's also kind of a fun mechanic--it makes the game, in some sense, more immersive.