Nobunaga was making an argument. Every stone, every panel of gold leaf, every muraled ceiling in the seven-story 天守 (tenshu) of Azuchi Castle was a line in the same declaration: Japan has a sovereign now, and he intends to stay.

Before Azuchi, Japanese castles were functional objects — military installations designed to be defended and, when necessary, abandoned. They were cold, dark, and deliberately impermanent, built for a world where power shifted constantly and a warlord's home could become his enemy's fortress within a season. Nobunaga rejected this logic entirely. He was 42 years old in the summer of 1576, master of the most powerful military force in Japan, and he had decided that the age of temporary commanders in temporary strongholds was over.

Construction began on Mount Azuchi, on the eastern shore of Lake Biwa in Omi Province. Nobunaga's retainer Niwa Nagahide oversaw the work. The location was chosen with the same precision Nobunaga applied to every military decision: near enough to Kyoto to exert pressure on the capital, far enough to be immune to its chronic fires; positioned to observe and control the approach routes from his three most dangerous rivals — Uesugi to the north, Takeda to the east, Mori to the west. The mountain itself became the foundation, its granite bones quarried and fitted without mortar into walls five and a half to six meters thick.

The 天守 (tenshu) — the main tower — rose seven stories. Six above ground, one below, the top floor an octagonal observation room looking out over Lake Biwa and the surrounding provinces. Kano Eitoku, Japan's greatest painter, was commissioned to decorate every floor of the interior. His murals depicted dragons, tigers, phoenixes, scenes from Chinese classical history — all rendered on walls and ceilings covered in gold leaf, so that the rooms glowed even in low light. Visiting daimyo arrived in functional fortresses and found themselves inside a work of art. Visiting Jesuit missionaries, accustomed to the palaces of Europe, compared it favorably to Rome.

Nobunaga also issued a municipal charter to the castle town in 1577, granting residents immunity from taxation and requiring all travelers on the Nakasendo highway to stop overnight — effectively forcing commerce into his city by law. By 1582, the town had grown to roughly 5,000 inhabitants, a city willed into existence by decree.

The castle was completed in 1579. Three years of construction, six years of existence. In June 1582, days after Nobunaga was assassinated at Honnōji Temple by his own general, Akechi Mitsuhide, Azuchi Castle burned. The cause is still disputed — deliberate destruction, the chaos of a leaderless garrison, an accident. All of it is gone. The great murals, the gold-leaf ceilings, the seven stories of Kano Eitoku's work: ash.

What remains is the influence. Toyotomi Hideyoshi looked at Azuchi and built Osaka Castle. Tokugawa Ieyasu looked at both and built Edo Castle. Every Japanese castle keep constructed after 1576 is, in some sense, an echo of Nobunaga's argument — the tenshu as symbol of permanent authority, the castle town as economic engine, the fortress as residence. The entire Edo period, three centuries of relative stability, was built on an architectural template introduced by a man whose own building lasted six years. The Azuchi-Momoyama period of Japanese history takes its name from this castle. An era named for a ruin.