Odani Castle

Odani Castle is a mountain fortress located in Nagahama City, Shiga Prefecture, Japan. Built during the Sengoku period, it was once part of the Azai District of Ōmi Province. Today, its ruins are designated as a National Historic Site. Standing on Mount Odani (also called Ibu-yama) at an elevation of about 495 meters, it is regarded as one of Japan’s five great mountain castles, renowned for its formidable terrain and tragic history.

The castle is most famously known as the residence of the Azai clan, a powerful Sengoku-period daimyo family, and as the stage where the fates of Azai Nagamasa and Oichi—the younger sister of Oda Nobunaga—intertwined. Taking advantage of the mountain’s ridgeline, Odani Castle was built along a long north–south axis, using ridges and valleys as natural defensive structures. It is believed that the original main bailey (honmaru) was located further north around the Ōdake Castle area. Under the rule of Azai Hisamasa and later his son Nagamasa, the fortress expanded to include multiple enclosures (kuruwa) and earthen ramparts.

During the Genki and Tenshō eras, Odani Castle endured a prolonged four-year siege by Oda Nobunaga, eventually falling. This battle led to the downfall of the Azai clan and marked Nobunaga’s expansion of power in northern Ōmi. After its fall, Nobunaga relocated his stronghold in northern Ōmi to lakeside Nagahama Castle, resulting in Odani Castle’s abandonment. Many building materials were dismantled and reused for the construction of Nagahama Castle, but remnants remain visible today, including massive stone walls near the Sannōmaru area.

At present, the ruins of Odani Castle preserve extensive remains such as the Honmaru, Sannōmaru, and Ōdake Castle sites. These ruins provide valuable insight into the defensive architecture of Sengoku-period mountain fortresses and their historical significance.