As usual with your games, the atmosphere is on point. I like the idea you had, but I feel like it stood itself in the way a bit. Unlike your other games, this one has absolutely no verticality or things to see from the distance. So searching for 20 keys is a lot. I do like what you did with the sound design to help finding them, that was really great. A suggestion that comes to mind is that it could have been a more enjoyable experience if there were either less keys, OR 20 keys in the maze, but you'd only need to find 10-15 of them to progress (the others disappear as soon as you have that amount, the player will never find out!) And then look for the other keys in the second level which looked way more fun to explore, and gives great opportunities for shining references.
Went a bit on a rant, hope that's ok :)