The palace was finished in six months. It lasted ten years. In 784, Emperor Kanmu ordered the capital of Japan moved forty kilometers north of Heijo-kyo -- the city we now call Nara -- to a new site at the confluence of the Katsura, Uji, and Yodo Rivers. The location was chosen for reasons both practical and political: three rivers meant easy transport of goods by boat, natural spring water on a gentle slope meant the city would flush itself clean, and a minor plateau fifteen meters above the surrounding streets meant the emperor could literally look down upon his subjects. But Nagaoka-kyo was cursed almost from the start. Within a year of its completion, the man who had chosen the site was murdered. The emperor's own brother died in exile, harboring a grudge. And then the dead prince's spirit began its revenge.
Emperor Kanmu had good reasons to leave Heijo-kyo. The old capital could only be supplied by land routes, an increasingly serious logistical problem. Its main seaport, Naniwa-tsu on the Seto Inland Sea, had silted up and was becoming unusable. The Buddhist clergy of Nara's great temples -- Todai-ji chief among them -- had grown deeply entangled in court politics, wielding influence that rivaled the emperor's own. And Kanmu wanted to build ties with the powerful toraijin immigrant clans of southern Yamashiro Province, from whom his own mother was descended. He consulted closely with Fujiwara no Tanetsugu, and it was no coincidence that Nagaoka happened to be Tanetsugu's family home and a stronghold of the Fujiwara clan. The new capital offered everything Nara lacked: river transport, a new route via the Kanzaki River toward Lake Biwa, fresh water flowing naturally through the streets, and separation from the monks who had made governing from Nara increasingly difficult.
New Year's ceremonies were held at the Nagaoka-kyo palace on January 1, 785 -- meaning the entire imperial palace was completed in roughly six months from the start of construction. The speed was remarkable, but it came with a calculated political decision. Normally, Japanese capitals were built by dismantling the old palace and relocating its buildings. But Kanmu left Heijo-kyo's structures intact, disassembling the Naniwa-kyo palace instead for building materials. This was a concession to opposition: the people of Nara would not watch their city torn apart. Every house in Nagaoka-kyo was built with its own well, and running water from the roadside was channeled into homes to carry away waste. The natural spring water flowed southeast through the gently sloping city and washed filth into the river. Archaeological excavations have confirmed these engineering details, revealing a city designed with sophisticated infrastructure that addressed the sanitation failures of Nara.
In September 785 -- less than a year after the capital's completion -- Fujiwara no Tanetsugu, the man who had championed Nagaoka-kyo's location, was assassinated. Officials associated with Todai-ji were implicated. Emperor Kanmu's younger brother, Imperial Prince Sawara, was imprisoned and exiled for his vocal opposition to the move. Sawara died en route to his place of exile, still harboring a fierce grudge. In the years that followed, catastrophe piled upon catastrophe: famine and epidemics struck in 792, the Empress and several of Kanmu's close relatives died in succession, the main hall of Ise Grand Shrine was set ablaze, and the Crown Prince fell ill. An onmyoji -- a practitioner of yin-yang divination -- declared that these disasters were the work of Prince Sawara's onryo, his vengeful spirit. Rituals were performed to appease the dead prince, but the spirit would not be calmed.
The very geography that had made Nagaoka-kyo attractive became its undoing. Despite efforts to pacify Prince Sawara's spirit, heavy rains brought devastating floods. The three rivers whose confluence had promised easy transport now inundated the capital. Wake no Kiyomaro, the aristocrat tasked with flood control, proposed what must have seemed unthinkable: abandon the capital entirely. A new site was selected to the northeast in 793, and in 794 the capital officially relocated to Heian-kyo -- the city that would become Kyoto and serve as Japan's capital for more than a thousand years. After the move, the former Nagaoka-kyo area passed to the domain of Sugawara no Michizane, and the Nagaoka Tenman-gu shrine was built after his exile in 901. The name 'Nagaoka' survived as a place name, but the actual site of the palace was lost for centuries.
It was not until 1954 that the palace site was rediscovered, buried beneath the modern cities of Muko and Nagaokakyo, the town of Oyamazaki, and the Nishikyo-ku ward of Kyoto. The ruins correspond to a 4.3-by-5.3-kilometer area spanning all four municipalities. Extensive archaeological excavations have been conducted since, revealing the foundations of the Daigokuden -- the Great Audience Hall -- the Goden, and the Chodo-in administrative complex. The palace ruins in the Kaidecho neighborhood of Muko were designated a National Historic Site in 1964, with the protected area expanded in 2016. Today, reconstructed markers and interpretive displays trace the outlines of halls where New Year's ceremonies were held just once before a prince's ghost drove an empire to find a new home.
Located at 34.94°N, 135.70°E in the suburban area between Kyoto and Osaka, spanning the modern cities of Muko and Nagaokakyo. From altitude, the site lies in a flat river plain at the confluence of the Katsura and Uji Rivers forming the Yodo River -- the same geographic feature that attracted Emperor Kanmu in 784. The archaeological park is modest, set within dense suburban development. Best viewed at 3,000-5,000 feet AGL. Osaka International Airport (RJOO) lies approximately 20 nautical miles to the south. The Yodo River corridor is the dominant landscape feature, with central Kyoto visible approximately 10 kilometers to the northeast.