Name in Japanese: 小谷城跡
Pronunciation: odani jō ato

Odani Castle was an important mountain fortress in the Warring States period. Its ruins are found to the north of today’s Nagahama, on the southern ridges of Mt. Odani at an altitude of about 495 m. The mountain is an enjoyable climb in itself, but if you understand the tragic history of the location, it makes the hike so much more rewarding.

Odani is counted as one of the five great mountain castle sites in Japan, and its ruins are designated a national historical site. This Warring States period castle was developed over three generations by the Azai clan, and it was the scene of the tragic end of this Ōmi clan at the hands of warlords from neighbouring Aichi. Known as an impregnable mountain castle, it fell after being besieged for four years by Nobunaga Oda. After that, it was abandoned, and Nagahama Castle became the seat of power in northern Ōmi. At the castle site today, you can still see earthwork enclosures and stone walls that were innovative at that time.

The site is a horseshoe shape. At its base is a carpark and the Odani Castle Sengoku Historical Museum sitting in the flat area between the steep ridges on either side. Before exploring the mountain castle, it’s best to visit the museum where dioramas and panels explain the structure of the castle and introduce the characters who fought over it.

A footpath goes up the ridge on the right, passing through several different kinds of forest. After a significant walk, you come to enclosures of various sizes and structure. Signboards show illustrations of how the enclosures looked in the past. The large stone walls reflect the growing power of the Azai clan as they gradually rose from local warlords to daimyō with influence on a national scale. The main keep of the castle was roughly halfway up the ridge, and further enclosures continue up the ridge, around the curve of the horseshoe at the highest point, and down the left ridge.

Some interesting sights are the stone-lined baths where horses were washed, and a large flat-topped stone on which the head of a general was exposed. Imai Hidenobu was suspected of spying for the Rokkaku clan and was killed by Azai Sukemasa at Hijiyama Shinshō-ji Temple in Nagahama.

The history of Odani Castle

During the Warring States period, the Azai clan were originally vassals of the Kyōgoku clan of northern Ōmi. But as the result of a dispute over succession in the Kyōgoku clan, Sukemasa emerged as the new leader and the Kyōgoku became subordinate to the Azai.

Thereafter, the Azai clan were in bitter conflict with the Rokkaku clan in southern Ōmi, with both clans inflicting defeats on each other. Azai Sukemasa completed Odani castle around 1525. Rokkaku Sadayori attacked it in 1538, and Sukemasa temporarily fled the castle to Gifu. He was able to return, but the situation was precarious and Sukemasa’s successor, Hisamasa, became the vassal of the Rokkaku.

However, at the age of fifteen, Hisamasa’s son Nagamasa rebelled against the Rokkaku and allied with Asakura Yoshikage of Fukui to the northwest and forced his father to retire as leader of the clan. Hisamasa was exiled for a while to Chikubu Island in Lake Biwa. To cement his position further, Nagamasa married Oichi, younger sister of Oda Nobunaga of Aichi to the northeast, establishing the Azai as a regional power.

Nevertheless, Nobunaga’s ambition was to unite Japan under his own rule and he soon went to war with the Asakura. The Azai sided with their old ally the Asakura against Nobunaga, setting the stage for the extinction of both allies.

At the Battle of Anegawa about five kilometers south of Odani Castle in 1570, the forces of Oda and Tokugawa defeated the Azai-Asakura army. Nobunaga was unable to destroy the castle immediately since it was so strong, but he built Yokoyama Castle on the south bank of the Ane River and placed his vassal Kinoshita Hideyoshi (later Toyotomi Hideyoshi) there to watch over Odani.

The Azai army sallied out from Odani Castle occasionally to attack the Oda forces, but the Azai generals holding Sawayama and other castles gradually switched sides. In 1573, the guardian of Yamamotoyama Castle, close to Odani, was persuaded by Hideyoshi to turn coat allowing the Oda forces to besiege Odani Castle directly. Nobunaga departed Gifu Castle in the middle of the night and succeeded in blocking the route of the Hokkoku Highway from Fukui to Odani Castle, preventing Asakura Yoshikage’s 20,000-strong army from entering Odani Castle. Instead, they set up camp outside the castle in Kinomoto.

On August 12, a storm struck the area. Nobunaga took this opportunity to take one of the subsidiary castles with the guidance of a turncoat. Furthermore, the next day, the Asakura army, regarding the situation as unfavourable, withdrew. Nobunaga assaulted the withdrawing Asakura at once, inflicting a devastating defeat. Leaving behind his heir Nobutada with a party of soldiers, Nobunaga attacked Fukui, completing the destruction of the Asakura clan. He returned to Ōmi on August 26. The following day, Hashiba Hideyoshi’s army swept down the steep slope of Shimizu Valley to attack the Kyōgoku enclosure of Odani Castle, and succeeded in separating Nagamasa, who defended the main enclosure, and Hisamasa, the father of Nagamasa, who defended a lesser enclosure. Hideyoshi took that enclosure the same day, and Hisamasa committed suicide.

Nagamasa secretly sent his ten-year old son, Manpukumaru, out of the castle under the protection of a vassal. He also sent his wife and three daughters out to the protection of Nobunaga who was Nagamasa’s brother-in-law. At that time, Nobunaga offered Nagamasa a pardon and relocation to another domain, but Nagamasa opted to fight on.

The main enclosure also fell, and on September 1, Nagamasa killed himself at the Akao residence in the main enclosure. It was the end of the Azai clan. Manpukumaru was quickly apprehended and was executed by impaling at Sekigahara. Nobunaga ordered the heads of Nagamasa and Asakura Yoshikage displayed in Kyōto, then had their skulls lacquered and made into cups.

Hashiba Hideyoshi was awarded large swathes of Azai territories, but Hideyoshi disliked Odani Castle, which is far from Lake Biwa, and in 1575, he built a new castle in today’s Nagahama facing the Hokkoku Kaidō road with a harbour on Lake Biwa. Odani Castle was abandoned.

It was designated a national historic site in 1937.