The palace exists because Japan lost a war. In 660 AD, the Korean kingdom of Baekje -- a close ally of the Yamato court -- fell to a coalition of Silla and Tang dynasty China. Crown Prince Naka no Oe, the future Emperor Tenchi, sent an expeditionary force to restore Baekje, but it was crushed at the Battle of Baekgang in 663 AD. The defeat ended Japanese involvement on the Korean Peninsula for centuries and left the court terrified of a counter-invasion. Mountain-top fortresses went up across western Japan. Then Emperor Tenchi made a more radical decision: he abandoned the old capital at Asuka entirely and moved the seat of imperial power to the shore of Lake Biwa in Omi Province. The location offered strategic advantages -- access to the Sea of Japan coast, position on the Tosando and Hokurikudo highways -- and political freedom. In Asuka, entrenched factions resisted reform. On the lakeshore, Tenchi could build something new. The people of Asuka responded with riots and arson.

What Emperor Tenchi accomplished during those five years at Otsu proved more durable than the palace itself. It was here that the Omi Code was promulgated -- one of Japan's earliest comprehensive legal frameworks -- along with a family registry system that organized the population for taxation and governance. These reforms laid the groundwork for the ritsuryō system, the Chinese-influenced legal and bureaucratic structure that would define the Japanese state for centuries. Tenchi was not just relocating a court; he was remaking a government. The urgency born of military defeat drove him to centralize authority and modernize administration at a pace that the old Asuka establishment found threatening. The palace on Lake Biwa became the laboratory for a new kind of Japanese governance, one that would persist in various forms long after the building itself had been reduced to postholes and pottery fragments.

Emperor Tenchi died in 671, leaving behind a succession dispute that would tear his legacy apart. He had initially designated his brother, Prince Oama, as heir, but changed his mind in favor of his son, Prince Otomo. Prince Otomo took the throne but held it for less than a year. Prince Oama raised an army and marched on the capital in what became known as the Jinshin War -- one of the most consequential civil conflicts in Japanese history. The war ended with Prince Otomo's death and the Otsu Palace in flames. Prince Oama became Emperor Tenmu and moved the capital back to Asuka, as if the five years at Lake Biwa had been an interruption to be erased. The palace, built without roof tiles and intended perhaps as a temporary seat of power, left almost no physical trace. Over the following centuries, even its precise location was forgotten.

The search for the Otsu Palace became something of a national obsession, particularly during the nationalism of the late Meiji and Taisho periods. Theories multiplied whenever ruins were found in the general area -- the site of Sufuku-ji temple, the Minamishigacho temple ruins, and numerous other candidates were all proposed and debated. The breakthrough came in 1974 with the rescue excavation in Nishikiori. Subsequent digs revealed Sue and Haji ware pottery, a double corridor extending from the south gate to a large main hall, and remnants of moats and earthen ramparts. The layout resembled the nearly contemporary Naniwa Palace discovered in Osaka. The total site is estimated at 700 meters north-to-south by 400 meters east-to-west. Japan designated it a National Historic Site in 1979 and expanded the protected area in 2007. Today the ruins are preserved as an archaeological park in the western part of Otsu, a short walk from Omijingumae Station on the Keihan Electric Railway.