Sanuki Castle was a historic castle located in Sanuki, Futtsu City, Chiba Prefecture. During the Edo period, it served as the administrative center of the Sanuki Domain and played an important role as a political and military base.
From the Sengoku period through the Edo period, Sanuki Castle repeatedly became a battleground in struggles between powerful regional clans. Its surviving earthworks and other remains continue to provide valuable clues about the history and military strategies of the time.
Sanuki Castle was a hirayamajo, or castle built on a low mountain or hill, and is generally believed to have been constructed by the Mariyatsu clan, also known as the Mariyatsu-Takeda clan.
It is thought that Mariyatsu Zenpo, also known as Takeda Nakiaki, served as the castle’s lord sometime between the Eisho era and 1537. During this period, members of the Kato family served as castle deputies and were involved in governing the surrounding area.
The Kato family was regarded as descendants of Kato Nyudo Bonken, a retainer of Takeda Nobunaga who appears in the medieval chronicle Kamakura Okagami. Even after the decline of the Mariyatsu clan, the Kato family remained an influential local family and continued to serve successive rulers.
After 1537, Sanuki Castle came under the control of the Satomi clan of Awa Province, with Satomi Yoshihiro using it as an important base. Together with Kururi Castle, it served as a frontline stronghold against the Mariyatsu clan and the Later Hojo clan. There was also a period when the Later Hojo clan is believed to have occupied the castle and stationed a castle deputy there.
One tradition holds that Sanuki Castle was built by Takeda Yoshihiro during the Onin era in the late 15th century. Another theory suggests that the Soshu Nagao family, mentioned in the Kamakura Okagami, constructed the castle. As a result, there are several theories concerning the castle’s exact origins.
In 1537, an internal conflict broke out within the Mariyatsu clan. Sanuki Castle, which was occupied by Mariyatsu Nobutaka, was attacked and captured by forces associated with Ashikaga Yoshiaki and Satomi Yoshiaki.
In 1545, Hojo Ujiyasu captured Sanuki Castle. The following year, in 1546, Satomi Yoshihiro recaptured it. From then onward, control of the castle became a major point of contention between the Satomi clan and the Later Hojo clan.
In 1559, Hojo forces once again took control of the castle, and Ashikaga Yoshikane, the Koga Kubo, established a temporary residence there. In 1563, however, the Satomi clan recaptured Sanuki Castle.
The castle subsequently became connected with several major events in the history of the Boso Peninsula, including the Battle of Mount Mifune and the death of Satomi Yoshihiro. Its strategic location made it an important military stronghold throughout the Sengoku period.
Following Toyotomi Hideyoshi’s Odawara Campaign in 1590, Sanuki Castle was taken over by Naito Ienaga, a retainer of Tokugawa Ieyasu. The castle then became the administrative center of the Sanuki Domain.
Over the following centuries, the domain changed hands several times as different feudal lords were appointed. The castle’s history as a feudal administrative center eventually came to an end with the abolition of the feudal domains and establishment of prefectures in 1869.
Sanuki Castle was built at the edge of a hill between the Kitakami River and Some River, taking advantage of the natural terrain to create a strong defensive position.
To the east of the castle grounds lies a valley known as Jinbaya. The valley formed part of the castle’s natural defenses and was incorporated into its overall defensive layout.
The central part of the castle was organized around the Honmaru, or main enclosure, with the Ninomaru and Sannomaru extending toward the southwest. Several additional fortified enclosures were arranged along the ridgelines.
The Honmaru included defensive features such as gates, earthen walls, and dry moats. Many of the surviving structures are thought to reflect renovations carried out during the period when the Satomi clan controlled the castle.
A number of important remains survive on the hilltop, including dry moats, stone walls, earthen embankments, and wells. These features allow visitors to imagine the appearance and defensive function of the medieval castle.
Some of the dry moats display characteristics associated with Satomi-style castle construction, while the surviving stonework is considered relatively unusual for the Kazusa region. Together, these remains provide valuable evidence of the castle-building techniques used during the Sengoku period.
Today, the former Sanuki Castle site is being developed as a place where visitors can explore the area’s history. It is particularly appealing to people interested in Japanese castles, medieval history, and traditional military architecture.
Walking through the site and observing the remaining moats, earthworks, stone structures, and natural terrain offers visitors an opportunity to imagine the conflicts and political changes that took place here from the Sengoku period through the Edo period.
Sanuki Castle is an important historic castle site associated with several major powers of the Boso Peninsula, including the Mariyatsu, Satomi, and Later Hojo clans. Its surviving remains and strategic location provide valuable insight into the military architecture and political struggles of medieval Kanto.
The combination of natural defenses, fortified enclosures, dry moats, stonework, and earthen walls makes the site particularly interesting for those exploring Japan’s castle heritage.
For travelers interested in Japanese history and castle ruins, Sanuki Castle offers a chance to walk through a landscape shaped by centuries of political and military change and to discover a lesser-known chapter of the Boso Peninsula’s past.