Enryakuji (延暦寺), a monastery on Mount Hieizan (比叡山) overlooking Kyoto, was founded during the late eighth and early ninth centuries by Saicho (最澄: 767-822 AD), also known as Dengyo Daishi (傳教大師), who introduced the Tendai (天台; Chinese tian1 tai2) sect to Japan from China. One of the most significant monasteries in Japanese history, it served as (and still is) the headquarters of the Tendai sect, the Buddhist sect that was popular among the aristocracy of the time and served as foundation for a number of later sects including the Pure Land (浄土), Zen (禅), and Nichiren (日蓮) sects.
With the support of the Emperor Kammu (桓武天皇 also spelt "Kanmu"), Saicho ordained a hundred disciples in 807. Maintaining a strict discipline on Mt. Hiei, his monks lived in seclusion for twelve years of study and meditation. After this period of study, the best students were retained in positions in the monastery, and others graduated into positions in the government and court. At the peak of its power, Enryakuji was a huge complex of as many as 3000 sub-temples and a powerful army of warrior monks (僧兵) who were occasionally engaged in power struggles with other monasteries and political leaders. In the tenth century, succession disputes broke out between Tendai monks of the line of Ennin (圓仁) and Enchin (圓珍; 814-891). These disputes resulted in opposing Tendai centers at Mount Hiei, the sammon (山門; 'Mountain Order') and at Miidera, the jimon (寺門; 'Church Order'). Warrior monks were used to settle the disputes, and Tendai leaders began to hire mercenary armies who threatened rivals and even marched on the capital to enforce monastic demands.
As part of a program to remove all potential rivals and unite the country, Shogun Oda Nobunaga (織田信長) ended this Buddhist militancy by attacking and razing the huge temple complex on Mount Hiei, including Enryakuji, and destroying the monastic communities.
Enryakuji consists of three sections: the Todo (東堂; East Hall, also the quarters where the former head priest resides), the Saito (西堂; West Hall, also the abode of a retired head priest from a separate monastery), and the Yokawa (横川). The monastery's most important buildings are concentrated in the Todo.