The real strength of this is the atmosphere it creates. The music is beautiful and the art is incredible, I'd need a 48 hour jam just to draw one of those bosses! Sound effects were simple, but effective, and the theme was well explored.
I have a few criticisms though, as any good feedback does.
1) The gameplay feels quite repetitive by the time you reach the 5th or 6th boss, maybe adding in some different attack types would help. It's not too challenging once you figure out the mechanics of wall-jumping over the boss to attack from behind, but some kind of ranged attack (probably with a limit) would really improve the gameplay I think.
2) The absence of a player healthbar was frustrating, I died a few times because I just had no idea how little health I had left before I went in to attack. This is a shame really, because the boss's healthbars look/work great.
Overall, this is really well made, and I had a lot of fun playing it. Great music, great art, and interesting bosses.