Gensokyo (幻想郷, literally Land of Illusions or Land of Fantasy; more accurately romanized as Gensōkyō or Gensoukyou) is the fictional land that the entire Touhou series takes place in. Its culture vaguely resembles that of feudal Japan, with a lot of folktale elements added on.
Gensokyo was originally a desolate, haunted region of Japan ages ago. Theyoukai that lived there began to terrorize the surrounding lands, and thus powerful, heroic humans were sent to exorcise and exterminate them. The off and on battle between humans and youkai continued on until 1884 A.D., when Gensokyo was sealed off from this increasingly scientific and skeptical world with the creation of the Hakurei Border. There has been little contact between those societies ever since. Today, the only known gateway between Gensokyo and this world is the Hakurei Shrine, which stands in isolation along the border on distant mountains of the far east.
Gensokyo is populated mainly by youkai, but a decent human and rabbit population lives there as well. Some of its inhabitants went there to hide, to escape, to find shelter when no one and nowhere else would accept them. Many of them just like the natural darkness.
Gensokyo, the area encompassed by the Hakurei Border, is a landlocked territory at least some tens of kilometers in diameter, although its exact size has not been mentioned in canon materials. Notable geographic features include the Youkai Mountain with communities of kappa at its base, tengu at its peak, and the Moriya Shrine at a lake near the top; the Forest of Magic; the Bamboo Forest of the Lost where Eientei is located; the Misty Lake surrounding the Scarlet Devil Mansion; the Human Village, which is assumed to be in a fertile valley; and the Sanzu River separating Gensokyo from Higan.
Although passing through the Hakurei Border to the outside world is virtually impossible for anyone besides Yukari Yakumo, it is significantly easier for Gensokyo denizens to access domains of gods, demons, and dead souls which are not strictly part of Gensokyo itself. These include Higan, the Netherworld, Makai, the Celestial realm seen in Scarlet Weather Rhapsody, and the underground abandoned Hell where Subterranean Animismis set.
Cage in Lunatic Runagate Chapter 4 reveals that the Youkai Mountain preserves the original appearance of Mount Yatsugatake before Sakuya-hime, the goddess of Mount Fuji, tore it down out of jealousy. In the outside world today, the remnants of the mountain can be seen as the Yatsugatake mountain range in Yatsugatake-Chūshin Kōgen Quasi-National Park between Nagano and Yamanashi Prefectures, providing an indication of where Gensokyo would be located in Japan. Lake Suwa (諏訪湖, Suwa-ko), which Suwako Moriya was named after, is located nearby.
The Gensokyo calendar dates from its sealing in 1884, and is based on the phase of the moon- due to calendar drift, it runs approximately two months earlier than ours.
Note that although the Windows games seem to follow a real-time progression as shown in this timeline, the human protagonists, Reimu Hakurei and Marisa Kirisame, don't apparently age beyond their mid-teens.
It is said, that sometimes or more recently a Man with a Military coat along with a Few others usually visit this Illusion's based Land it more seems like they are apart of Trinity Though it seems they are a bunch of abnormal beings one of being a Tengu...maybe it holds more then we know.