Saicho, a young monk dissatisfied with the Buddhism he had studied in the capital, built a small hermitage on Mount Hiei in 788 CE and, after a period of study in China, returned to found it formally as Enryaku-ji, headquarters of the Tendai school he had brought back with him, built around the Lotus Sutra and a comprehensive, syncretic reading of Buddhist teaching. Saicho spent much of his later life petitioning the court for the right to ordain monks on Mount Hiei using a Mahayana ordination platform independent of the older Nara schools, a dispute settled only after his death in 822, when the kaidan, the ordination platform, was finally authorized. That victory mattered more than its narrow procedural content suggests, because it made Mount Hiei the training ground of choice for ambitious monks for the next four centuries, largely free of Nara's institutional control.
Honen, who founded Pure Land Buddhism's Jodo-shu; his student Shinran, who founded Jodo Shinshu; Eisai, who brought Rinzai Zen to Japan; Dogen, founder of Soto Zen; and Nichiren, founder of the school that bears his name, all trained on the mountain before breaking away to found their own schools, so that nearly every major current of later Japanese Buddhism traces at least one root back to Mount Hiei's monastery, even where the schools that grew from it later stood in sharp doctrinal opposition to Tendai itself and to each other.