Youkai Mountain (Youkai no Yama, 妖怪の山) is reputedly the greatest mountain in Gensokyo. At the base of it live the Kappa, such as Nitori Kawashiro, and at the top are the Tengu. It's actually such a dangerous place full of youkai, some of which don't like humans, that humans are normally unable to come here.
This mountain is protected by the white wolf Tengu, such as Momiji Inubashiri, who patrol it. Also among the tengu living here is Aya Shameimaru, as well as other newspaper-writing tengu. The tengu chief is Lord Tenma.
Lately a new arrival from the outside world, the Moriya Shrine, has appeared on the mountain along with a lake. Sanae Kochiya, Kanako Yasaka, and Suwako Moriya all live at or near here. Both the Kappa and tengu have come to accept Kanako as a resident god and now live in peace.
According to Perfect Memento, Suika Ibuki can also be found here.
Revealed by chapter four of Cage in Lunatic Runagate to be the Yatsugatake mountain range from the distant past, giving the only indication as to the real-world location of Gensokyo.