Scarlet Devil Mansion (often abbreviated to SDM) is a prominent place in Gensokyo.
The owner of the mansion is Remilia Scarlet, and the residents include Flandre Scarlet(her sister), Sakuya Izayoi(head maid), Hong Meiling(door guard), Patchouli Knowledge(head librarian),and Koakuma(assistant librarian.)
There is also a large number of maids, and possibly other staff like cooks ect.
The mansion has a large library (where Patchouli knowledge tends to reside) and a basement (where Flandre Scarlet resides).
In the context of Yukkuri, Scarlet Devil Mansion is the scene for stories involving Yukkuri Remilias and Yukkuri Flandres, and occassionally Yukkuri Sakuyas.
A bodied Yukkuri Remilia(or a whole group of them) lives in SDM and is pampered by Sakuya.
Sometimes there is a Yukkuri Flandre, whose main hobby is torturing the Yukkuri Remilia.
Wild Yukkuri Sakuyas may travel to SDM hoping to meet the real Remilia.
Yukkuri Remilias may escape to SDM hoping to get protection from the real Sakuya.
Occassionally SDM may be the scene of magical or scientific "experiments" conducted on the Yukkuri by Patchouli Knowledge.