The precise year Kannonji Castle was built remains unknown, but sometime during the 14th century the Rokkaku clan -- a cadet branch of the Sasaki clan who had risen to prominence during the Kamakura period -- chose the ridged peak of Mount Kinugasa as their seat of power. The Rokkaku served as shugo, or military governor, of Ōmi Province, though they effectively controlled only its southern half. Their northern rivals, the Kyogoku clan, held the rest. A Buddhist temple called Kannonshō-ji already occupied the mountain's upper slopes, and the castle took its name from this ancient sanctuary. Originally called Sasaki Castle, the fortification was rebuilt during the Ōnin era between 1467 and 1477, a period of devastating civil war that tore Japan apart.

Kannonji Castle changed hands three times during the Ōnin War alone -- twice in clashes between the Rokkaku and the Kyogoku, and a third time in a fratricidal struggle between rival branches of the Rokkaku themselves. Stability returned under Rokkaku Takayori, who died in 1520 after reunifying the clan by seizing estates from nobles and Buddhist temples with the backing of the Muromachi shogunate. His son Rokkaku Sadayori, who lived from 1495 to 1552, transformed the castle town of Ishidera at the mountain's base into a thriving commercial center by abolishing the restrictive local guild system. He sheltered Shogun Ashikaga Yoshiharu after Yoshiharu was expelled from Kyoto, and extended Rokkaku influence over the powerful Azai clan to the north. During the Kōji era of 1555 to 1558, the stone ramparts were enlarged to accommodate small cannons -- an early sign that Japan's age of the sword was giving way to gunpowder.

In September 1568, Oda Nobunaga marched westward toward Kyoto with an army intent on remaking Japan. The Rokkaku, loyal to the Ashikaga shogun, attempted to block his advance from Kannonji Castle. It was a fatal miscalculation. Nobunaga's general Niwa Nagahide crushed the Rokkaku forces so completely that by the time Nobunaga himself arrived at the castle gates, there was no one left to resist him. He entered Kannonji Castle unopposed and in triumph, then continued his march to the capital. The defeated lord Rokkaku Yoshikata fled south into the mountainous Kōka region, where he waged a guerrilla resistance from Mikumo Castle for several years. He never recovered his ancestral stronghold. Nobunaga kept Kannonji Castle garrisoned even after completing his grand new fortress at nearby Azuchi Castle, recognizing the older site's enduring strategic value.

The end came suddenly. When Nobunaga was assassinated in 1582 at the Honnō-ji incident in Kyoto, the power vacuum ignited chaos across his domains. Kannonji Castle was attacked and burned, possibly by local peasants turned looters who also torched the magnificent Azuchi Castle nearby. Two centuries of military history went up in flames in a single convulsion of opportunistic violence. Today the ruins sit quietly among cedar forests on Mount Kinugasa, protected as a National Historic Site since 1982. Hikers reach them via a 25-minute walk from JR Azuchi Station, climbing first past the ancient temple of Kuwanomi-dera, then through groves of Japanese cedar to the castle grounds. From the main courtyard, a trail continues to the neighboring Kannonshō-ji temple, where the panoramic view across the plains and Lake Biwa reveals exactly why the Rokkaku chose this mountain -- and why Nobunaga wanted it.