Gassan-Toda Castle

Gassan-Toda Castle (Gassan Toda-jō) is located in Tomita, Hirose-chō, Yasugi City, Shimane Prefecture. It was built atop Mount Gassan at an elevation of about 183.9 meters and is classified as a mountain castle. Originally the residence of the Tomita clan of the Izumo Genji lineage, it later came under the control of the Kyōgoku clan, who entrusted it to their deputies, the Amago clan. Over time, the Amago grew in power, drove out their overlords, the Kyōgoku, and rose as Sengoku daimyō, establishing this castle as their stronghold. In 1934, the ruins of Gassan-Toda Castle were designated a National Historic Site, and the surrounding area was incorporated into the Shimizu-Gassan Prefectural Natural Park.

Between 1396 and 1566, the castle served as the Amago clan’s base of power and held immense strategic importance. Under Amago Tsunehisa, the foundation of Izumo was solidified, and his grandson, Amago Haruhisa, expanded their influence across eight provinces of the San’in and San’yō regions, becoming a powerful daimyō. Built along the natural contours of the mountain, the castle’s rugged terrain made it extremely difficult to capture, earning it the reputation of being an impregnable “castle in the sky.” It was one of the most representative fortresses in the San’in region. However, after prolonged conflict with the Mōri clan, the Amago were eventually defeated, and the castle fell under Mōri control.

After the Battle of Sekigahara in 1600, the castle came under the rule of the Horio clan. Yet in 1611, when Horio Tadaharu relocated to Matsue Castle, Gassan-Toda Castle was abandoned. Nevertheless, until its abandonment, it had remained the foremost fortress of the San’in region and was regarded as one of the “Five Great Mountain Castles of Japan.”