I think this really captures the essence of a good horror game: a game where things don't seem too scary at first, but the further you go, the spookier it gets. The gradual introduction of the game's mechanics and dangers is extremely effective - I remember being taken aback by the bell demon's move and genuinely panicking, followed by intense relief when it ended just as my Sanity was going down to 10!
The only complaint I have is that even with the mechanics, the game's pacing is still a bit too slow and the overall difficulty doesn't ramp up much for a 100-room game. It would definitely benefit from more monsters and/or a sprint mechanic! Also, I'm not sure how the bell demon's move is programmed. Is it designed to end after you escape through a few more rooms? If so, it's incredibly clever and I'd love to see more of such a mechanic.
The visuals, audio and overall atmospheric design of the game really, really work in its favour. Youmu's face expressions and pose, the sound cues for the monsters (Mima especially), and the art style which is both simple and powerful... all of these elements came together to deliver a fine survival horror experience. Genuinely impressed this was made in 3 days.
Good job on the game!!!