Enryakuji, the headquarters of the Tendai sect of Japanese Buddhism, is a massive temple complex on Mount Hiei north west of the city of Kyoto, Japan. Perhaps the most important and and powerful Buddhist temple in all of Japanese History, Enryakuji remains an extremely important cultural and historical site to this day.
Enryakuji was founded by the monk Saicho (posthumously known as Dengyō Daishi) in the year 788 as the spiritual sanctuary for monks to study practice the Tendai branch of Esoteric Buddhism which he had just introduced to Japan after learning of it on a trip to China. Although Saicho originally chose the gorgeous mountaintop location with its serene mist-shrouded forests to get away from the hustle and bustle of worldly life in the Kyoto, its strategic location overlooking Japan's Imperial capital would later allow the ever more worldly monks of Enryakuji to exercise a disproportionate influence on Japanese politics.
For one thing, Enryakuji's proximity to Kyoto made it easy for Japan's ruling elite to patronize it. On the other hand, its location on top of a forbidding mountain with its treacherous slopes, trackless forests, and secret paths known only to the monks made it a natural fortress that was almost impossible to attack. Not surprisingly, the monks of Enryakuji gradually became increasingly wealthy and increasingly influential. At the height of its power in the mid-16th century, Enryakuji controlled a vast network of branch temples and lucrative private estates (shoen) all across Japan, and the main temple itself boasted over 3000 separate structures on Mount Hiei, not to mention an army of more than 7,000 warrior monks (or sohei).
At the slightest provocation, these restless warrior monks would come charging down the slopes into the center of Kyoto, disrupting commerce and causing mayhem until their demands were met. Of course no one could rule Japan without controlling the capital, and no one could control the capital without first reaching an understanding with the monks of Enryakuji. On more than one occasion the monks were even able to dictate who the next Emperor should be.
This increasingly came to be seen as a problem by the various nobles and warlords competing for control of Japan in the tumultuous Sengoku Era. Several attempts were made to attack or destroy Enryakuji, but all failed. The problem was that even if someone was able to somehow make it up the Kyoto side of the mountain with a large force, the monks would just fall back down the other side of the mountain to the heavily armed and fortified town of Otsu (which was firmly in the temple's pocket) and regroup.
Finally, in 1571, the crafty warlord Oda Nobunaga hit upon a solution. He raised a massive army, which he split in two, sending one half up the Kyoto side and one up the Otsu side (destroying the town of Otsu on the way) in a coordinated assault. Trapped in this pincer maneuver, the monks of Enryakuji were wiped out, and Nobunaga's forces burned the entire 3000-building complex to the ground, with the exception of a single tiny sub temple which they overlooked because it was located on the side of a cliff. Although the temple would later be rebuilt, it would never again exercise the kind of military or political influence it once had.
But beyond just worldly influence and importance, the spiritual influence of Enryakuji should not be understated. As the most famous and powerful temple in Japan, Enryakuji became the Harvard or Oxford University of training Buddhist monks. Basically any monk who was anybody had to go to Enryakuji for training, and thus pretty much all of the most famous Buddhist monks in Japanese history trained there, including founders of later Buddhist sects such as Shinran and Nichiren.
Today Enryakuji is a quiet, contemplative place, whose austere wooden buildings surrounded by deep woods provide a nice change of pace from Japan's boisterous city life. An easy day trip from Kyoto or Osaka, the mountain is accessible by bus, cable car, or ropeway, and can also be hiked up. Most visitors come up the Kyoto side, but going up the Otsu side is highly recommended, as the stunning views of Lake Biwa alone make the journey worthwhile. Once you reach the top, a shuttle bus can take you from place to place in the massive complex. Little trace remains of Enryakuji's violent and contentious past, but you will know that you are standing at the heart of what was once the most powerful temple in Japan.