Nobunaga chose his location with surgical precision. Mount Azuchi sat close enough to Kyoto that he could monitor the imperial capital and guard its eastern approaches, but far enough away that the constant fires and political upheavals of the city could not threaten his stronghold. The position also placed him squarely between his three most dangerous rivals -- the Uesugi clan to the north, the Takeda in the east, and the Mori to the west -- controlling the transportation corridors that connected them. Lake Biwa itself, Japan's largest freshwater lake, served as both a natural barrier and a supply route. Niwa Nagahide oversaw the three years of construction, marshaling laborers and stonemasons to build something Japan had never seen: a castle designed as much to impress as to defend. The all-stone walls were a revolutionary departure from the earthen fortifications of the Sengoku period.

The castle's interior was as radical as its exterior. Confucian figures decorated the walls, a deliberate signal that Nobunaga's ambitions had shifted from battlefield tactics to the art of ruling. Architectural historian Akira Naito concluded in 1976 that the tenshu -- the central tower -- featured an atrium rising from the basement to the fourth floor, possibly influenced by Jesuit visitors, with a Buddhist stupa at its center. The design remains debated among scholars, but the ambition is not. Nobunaga wanted awe. On New Year's Day in 1582, he opened the castle to visitors -- daimyos and their retainers, who were each instructed to bring 100 mon for the privilege. So many crowded in that part of a stone wall collapsed, injuring and even killing some guests. Nobunaga himself waited at the end of the tour to collect the admission fees in person. It may have been the first ticketed castle tour in Japanese history.

On June 21, 1582, Nobunaga was attacked and killed by his own general, Akechi Mitsuhide, at the temple of Honno-ji in Kyoto -- one of the most famous betrayals in Japanese history. Within days, Mitsuhide's forces occupied Azuchi Castle. A week later, the castle was set ablaze. Who lit the fire remains uncertain: some accounts blame looting townspeople, others point to one of Nobunaga's own sons. The flames consumed everything -- the gilded tower, the Confucian paintings, the octagonal belvedere with its views across Lake Biwa. The castle that had taken three years to build was gone in hours. Mitsuhide himself would be dead within thirteen days, killed at the Battle of Yamazaki by Toyotomi Hideyoshi, who would go on to finish Nobunaga's work of unifying Japan.

Today, only the stone foundations remain, climbing the wooded slopes of Mount Azuchi in terraced layers that trace the ghost of Nobunaga's ambition. The site was designated a National Historic Site in 1926 and upgraded to a Special National Historic Site in 1952. It sits within the Biwako Quasi-National Park, and the Japan Castle Foundation named it one of Japan's Top 100 Castles in 2006. An approximate replica of the castle tower stands at Ise Sengoku Village, a samurai theme park, while the Nobunaga no Yakata Museum near the original ruins displays a full-scale reproduction of the top floors. But the real legacy of Azuchi Castle is invisible. Researchers agree that it served as the catalyst for all the great stone castles that followed -- Osaka, Himeji, Matsumoto -- transforming Japanese castle architecture from military practicality into political theater. The Azuchi-Momoyama period, named partly for this vanished fortress, marks the beginning of early modern Japan.