Thanks a lot. Glad you found the top hat and enjoyed a bath in the hot spring 😊
I would expect this to feel even more nostalgic on the DS :)
I used GB Studio to create the game and the parallax effect is one of the built-in options. So I can't say for sure what's running under the hood. But I'm pretty sure it's H-blank scrolling. There are not really any other options for the Game Boy to handle more complex parallaxes like this.