Iwabitsu Castle is a yamashiro-style (mountain) castle located atop Mount Iwabitsu in Higashiagatsuma, Gunma Prefecture, Japan. Recognized for its historical significance, its ruins have been protected as a National Historic Site since 2019.
Situated in northwestern Gunma Prefecture, the castle overlooked a key route through the Agatsuma River valley, a corridor connecting Takasaki to Shinano Province via the Shinshu Kaido and Torii Pass. This strategic location made it a vital stronghold for controlling regional travel and military movements.
The origins of Iwabitsu Castle remain uncertain, though it is believed to have been built by the Agatsuma clan in the 13th century. During the Muromachi period, it came under the control of the Saito clan, another local warlord. However, in the early Sengoku period, Sanada Yukitaka, with the backing of his overlord Takeda Shingen, captured the castle in 1563 after defeating the Saito clan, who had unsuccessfully sought aid from Uesugi Kenshin of Echigo Province.
Sanada Masayuki later used Iwabitsu Castle as his base to expand into northern Kozuke Province, capturing Numata Castle in 1579. He also fortified the surrounding jokamachi (castle town) with defensive structures, a strategy he later employed in the design of Shinpu Castle for the Takeda clan. After the fall of the Takeda in 1582, Masayuki planned to shelter Takeda Katsuyori at Iwabitsu, but Katsuyori was betrayed by his retainers and committed seppuku before reaching safety.
Under the Toyotomi regime, Iwabitsu and Numata were granted to Sanada Nobuyuki, while his father Masayuki retained control of Ueda Castle. Following the death of Toyotomi Hideyoshi, Masayuki remained loyal to Toyotomi Hideyori and Ishida Mitsunari, whereas Nobuyuki sided with Tokugawa Ieyasu. After the Tokugawa victory at the Battle of Sekigahara in 1600, Nobuyuki retained his holdings in northern Kozuke, but the one domain, one castle policy of the Tokugawa shogunate led to the dismantling of Iwabitsu Castle in 1615. The jokamachi survived as a post station but was relocated closer to the Agatsuma River.
Today, no structures from the original castle remain. Visitors can reach the site via a 30-minute walk from Gunma-Haramachi Station on the JR East Agatsuma Line, followed by a challenging 30-minute hike up the mountain to the former inner bailey.
In recognition of its historical importance, Iwabitsu Castle was included in the "Continued Top 100 Japanese Castles" list in 2017.
See also
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Kaminoyama Castle
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Imabari Castle
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Iwakuni Castle
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Gujo Hachiman Castle
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Amagasaki Castle
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Hiroshima Castle
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Fukuyama Castle
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Tiba Castle
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