Welcome to Gaia! ::

In Gensokyo, there's a somewhat large and deep lake.
Near the lake stands a western-style mansion painted in a conspicuous, dazzling scarlet. It's called the Scarlet Devil Mansion, for the vampire who lives there. The interior is based on scarlet and other dazzling colors. Still, it has few windows and the inside is somewhat dark, so the color isn't too hard on the eyes. Even so, the restless atmosphere from the whole mansion gives it a childish feel. Yes, the vampire who is the mistress is still a child.
In this western-styled mansion, they always hold various events. Still, most of them aren't announced in advance, so the few other people attend them other than the residents. However, there are a lot of maids. Only the staff of the Scarlet Devil Mansion seem to be able to enjoy them. Holding such events shows the character of the mistress, who's not elegant at all, like the color of the mansion.
I don't know when it showed up, but I believe it hasn't existed for long. Surely the building isn't new. It may be the one that originally stood in the outside world. If a building loses its existence from the outer world, it can sometimes become an illusionary one. I'm afraid in the outside world, such antique mansions may not exist anymore. Even if they exist, they may be imaginary buildings; just for display, and not for living in.
Inside the mansion lives the vampire and the maids serving her. The existence of the whole mansion in Gensokyo may show that the vampire and maid are the illusionary beings that do not exist in the outer world anymore. I guess it's possible to see that an event occurred in the outside world which would bring it here.