I'm really not sure what it is, but something about this level just works. I'm the first person against this kind of edgy text and sudden effects to make levels feel more creepy, but the reason it works here is it's doing the opposite. It's not using scary art or jumpscares or anything remotely similar. It just uses a single core object: the smiley face. And that makes it feel so much more genuine. The whole theme combined with the silent song as a whole just makes it naturally mysterious without trying hard.

I still cannot put my words into why I like this level as much as I do, but it's been stuck in my head for years and memorability is one of the things I consider most important in any level.