Azuchi-jo Castle was one of the primary castles of Oda Nobunaga. It was built from 1576 to 1579, on the shores of Lake Biwa, in Omi Province. Nobunaga intentionally built it close enough to Kyoto that he could watch over and guard the approaches to the capital, but, being outside the city, his fortress would be immune to the fires and conflicts that occasionally consumed the capital. This location was also quite strategically advantageous, in managing the communications and transportation routes between his greatest foes - the Uesugi clan to the north, Takeda clan in the east, and Mori clan to the west.
Unlike earlier castles and fortresses, Azuchi was not intended to be solely a military structure, cold, dark, and foreboding. Nobunaga intended it as a lavish mansion, which would impress and intimidate his rivals, not only with its defenses, but with its lavish apartments and decorations, and flourishing town and religious life. The keep, called tenshukaku (or tenshu), rather than being the center of the castle's defences, was a seven-story building containing audience halls, private chambers, offices, and a treasury, as though it were a royal palace. In addition to being one of the first Japanese castles with a tower keep, Azuchi was unique in that its uppermost story was octagonal. In addition, the facade of Azuchi, unlike the solid white or black of other castles, was colorfully decorated with tigers and dragons.
There were five main militaristic features of Azuchi-jo Castle that differentiated it from earlier castle designs. Firstly, it was a massive structure, with the walls of the castle ranging from 18 feet to 21 feet in thickness. The second feature of Azuchi Castle is the predominant use of stone. The walls were constructed from huge granite stones fitted carefully together without the use of mortar. A third innovation of the Azuchi Castle was the high central tower, or donjon. The tower allowed for increased visibility for the use of guns against an opposing force. Builder's plans for the castle show the donjon to be 138 feet tall, with seven levels. Fourthly, Azuchi Castle had irregularly formed inner citadels. These inner citadels gave defenders ample defensive positions against intruders. The location of Azuchi Castle was also a novel feature. Whereas most Japanese castles found the most advantegous position was at the base of mountain surrounded by dense vegetation (which would allow cover for an enemy), Azuchi Castle was built on a plain to give a wide view of an approaching enemy.
Nobunaga desired a full castle town, and built well-defended homes for his generals, a Jodo-shu Buddhist temple called Jogon-in, and a number of homes for commoners a short distance away on the shore of the lake. However, he had trouble convincing people to move into these homes at first. In the summer of 1577, he issued a municipal charter, guaranteeing residents immunity from taxes, building or transport levies, and moratoria, and forced all travelers on the Nakasend¨ highway to stop in the town overnight for lodging, thus bringing business to his town's innkeepers. By 1582, the town's inhabitants numbered roughly 5,000.
In addition to welcoming many of Nobunaga's powerful political guests, such as Tokugawa Ieyasu and Niwa Nagahide, Azuchi castle also hosted an event in 1579 which has come to be known as the Azuchi religious debate (Azuchi shuron), taking place between leaders of the Nichiren and Jodo-shu sects of Buddhism.
In the summer of 1582, just after Nobunaga's death at Honno-ji Tempple, the castle was attacked by the forces of Akechi Mitsuhide, Nobunaga's betrayer. The castle was set aflame, though some accounts claim this might have been the work of looting townspeople, or of one of Nobunaga's sons. Mitsuhide, therefore, never managed to occupy the castle.
The Azuchi-Momoyama-era of Japanese history takes its name, in part, from this castle. All that remains of the castle today is the stone base. However, a reproduction of Azuchi, based on illustrations and historical descriptions, stands in Ise Sengoku Village, a samurai theme park near Ise. In addition, a full-scale replica of the top floors of the donjon is on display at the Nobunaga-no Yakata Museum near the original castle ruins.