Really clever idea!
I love the mice. The way they smile as they run away with the key. The way they don't return to their original position after stealing it, so that the level gradually gets easier as I deplete the mice. The one (two?) gems they guard where you don't actually need the key, so you can just walk past the mice keyless.
The music is excellent. I like the more and more epic versions that play as (1) I get an upgrade (2) I reach the final area (3) I get the ending. Very nicely done. And it's clever how you fade in and out with slight variations on the song depending on whether I have the key or not.
The final area is lovely and the ending is very satisfying.
(I lost the key to a bug in the red area, but restarted anyway because I was enjoying it and wanted to see where you were going.)