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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Amagasaki Castle
The founding year of Amagasaki Castle is traditionally considered to be 1617, when Toda Ujikané built his castle here, making it the administrative center of the Amagasaki Domain. However, as early as the Sengoku period, a fortress built by the Hosokawa clan already stood on this site. After the fall of Itami Castle in 1579, Araki Murashige—formerly a vassal of Oda Nobunaga who had rebelled against him—fled to this earlier castle.
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Hiroshima Castle
Mōri Terumoto (1553–1625) was the grandson and rightful heir of the renowned Mōri Motonari. When Terumoto became the head of the Mōri clan in 1571, he inherited vast territories covering a large part of the San’in and San’yō regions in western Honshū. In addition, the Mōri clan possessed the largest and most technologically advanced naval fleet of its time.
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Fukuyama Castle
After the defeat of Toyotomi Hideyori’s supporters in the Osaka Campaigns of 1614–1615, many clans in Japan still remained not fully loyal to the Tokugawa shogunate, especially in the western Chūgoku region. Mizuno Katsunari (1564–1651), a cousin of Tokugawa Ieyasu, became the first of the Tokugawa house’s close retainers, the so-called fudai daimyō, to be relocated to this strategically important area.
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Tiba Castle
The founder of the Chiba clan is considered to be Chiba Tsunesige (1083–1180), who in 1126 moved his residence to the Inohana area and built a strongly fortified stronghold there. Although Tsunesige himself came from the Taira clan, the Chiba clan later supported Minamoto no Yoritomo, the future founder of the first shogunate.
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Sunomata Castle
Oda Nobunaga, as part of his “final solution” to the conflict with the Saitō clan, conceived the construction of a fort in the Sunomata area, which was intended to serve as a forward base for an attack on Inabayama Castle (later renamed Gifu). Sunomata was a swampy area located between Ogaki Castle, a stronghold of the Oda clan, and Inabayama, the main fortress of the Saitō clan. Earlier attempts to build fortifications in this area, carried out by Oda generals Sakuma Nobumori and Shibata Katsuie, had all ended in failure. After that, the task was entrusted to the young vassal Kinoshita Tokichirō, who later became known as Toyotomi Hideyoshi.
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Shibata Castle
The exact time when structures first appeared on the site of the present-day castle is unknown; however, it is generally believed that the first fortified buildings were constructed here during the Muromachi period, when these lands were controlled by the Shibata clan.
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Okayama Castle
According to a number of accounts, the earliest fortifications on the site of present-day Okayama Castle appeared as early as the 14th century and were built by the Nawa clan. The Asahigawa River was used as a natural defensive barrier, protecting one side of the fortifications.
In the early 16th century, the Kanamitsu clan constructed a new castle here, which at the time was known as Ishiyama. In 1573, it came under the control of the Ukita clan, after which Ukita Naoie (1529–1582) launched large-scale reconstruction efforts. He did not live to see the work completed, and construction was continued by his son Hideie. -
Nagahama Castle
Hashiba, later known as Toyotomi Hideyoshi, received Odani Castle and the surrounding lands from Oda Nobunaga after the defeat of the Azai clan. However, Odani Castle was located high in the mountains, which made it poorly suited for the effective administration of the territory. For this reason, in 1575 Hideyoshi began constructing a new castle in the village of Imahama on the shore of Lake Biwa. Taking the character naga from Nobunaga’s name, he renamed both the village and the new castle Nagahama.
