Tatsuoka Castle

Tatsuoka Castle (Japanese: Tatsuoka-jō), also known as Tatsuoka Goryōkaku or Kikyō Castle, is located in present-day Taguchi, Saku City, Nagano Prefecture, Japan. Built during the final years of the Edo period, it served as the administrative center of the Tatsuoka Domain (also known as the Tanoguchi Domain). Because the domain held only 16,000 koku, it did not qualify to construct an official castle under the Tokugawa shogunate's regulations, and the complex was therefore officially designated as the Tanoguchi Jinya (administrative headquarters). Tatsuoka Castle is best known for its distinctive five-pointed star-shaped bastion design, making it, together with Goryōkaku in Hakodate, one of only two surviving star forts in Japan. Today, it is designated as a National Historic Site of Japan and was selected as one of the Top 100 Continued Japanese Castles in 2017.

Construction of Tatsuoka Castle began in 1863. That year, Matsudaira Norikata, lord of the Okudono Domain in Mikawa Province, petitioned the Tokugawa shogunate for permission to relocate his domain headquarters to Saku District in Shinano Province. Since most of the domain's territory was already located in the Saku region, and with the political situation becoming increasingly unstable during the final years of the shogunate, moving away from the Tōkaidō corridor to the domain's principal lands offered both administrative and strategic advantages. Fascinated by Western military science, Matsudaira incorporated the concept of the European-style bastion fort into the design of the new headquarters, selecting Tanoguchi Village as the construction site. Building officially commenced in 1864.

The entire complex was built at a cost of approximately 40,000 ryō. By 1867, the lord's residence and the principal buildings had been completed, while the stone walls and earthen embankments were largely finished. However, after Matsudaira Norikata assumed important positions within the Tokugawa government, his growing administrative responsibilities, combined with increasing financial pressures, prevented the project from being fully completed. As a result, Tatsuoka Castle remained unfinished according to its original design. After construction, the domain lord opened the new headquarters to retainers and local residents, allowing them to view this innovative fortress inspired by Western military architecture.

Following the Meiji Restoration, the Tatsuoka Domain was abolished, and most of the castle's buildings were dismantled and auctioned in 1872. Only part of the lord's residence, known as the Odaidokoro (kitchen and service quarters), was preserved by relocation and remains on the site today. The original moat was once filled in but was restored by local residents during the early Shōwa period. Beginning in 1875, the castle grounds were repurposed as a school campus, first for Shōyū School and later for Taguchi Elementary School, serving educational purposes for more than a century. After Taguchi Elementary School officially closed in 2023, a comprehensive restoration project was launched with the goal of gradually returning the site to its appearance during the early years of its completion.

Tatsuoka Castle covers an area of approximately 20,000 tsubo (about 66,000 square meters), making it roughly one-quarter the size of Goryōkaku in Hakodate. At its center stood the domain lord's residence, while the northeastern and southeastern bastions contained the main gate and service gate, respectively. Emergency exits were placed at the remaining re-entrant angles, and the complex also included samurai barracks, a gunpowder storehouse, a drum tower, and a shrine dedicated to successive domain lords. Today, visitors can still see surviving features such as the moat, stone walls, main gate, and the preserved Odaidokoro. Outside the fort, the Goryōkaku Exchange Center introduces the history and unique characteristics of Tatsuoka Castle.

Although Tatsuoka Castle adopted the outward appearance of a Western-style bastion fort, its defensive design did not fully conform to contemporary European military engineering principles. The fortress was equipped with only a single artillery battery, its moat was considerably narrower than those of typical European star forts, and the surrounding hills overlooked the entire complex, leaving it vulnerable to enemy artillery fire. Consequently, historians generally regard Tatsuoka Castle as an experimental adaptation of Western fortification techniques inspired by Matsudaira Norikata's interest in modern military architecture, rather than as a fortress intended for actual battlefield defense. This unique blend of traditional Japanese administration and Western military design makes Tatsuoka Castle an important historical landmark illustrating Japan's transition from conventional castle architecture to modern military fortifications during the closing years of the Edo period.