Saga Castle is located in Jonai, Saga City, Saga Prefecture. It is also known by its old name, Saga Castle (Sagajo), and was completed in the early Edo period as the residence of the Nabeshima family, the lords of the Saga Domain. The castle is also known by the nicknames “Submerged Castle” and “Turtle-Shell Castle.” It is a flatland castle featuring a complex combination of concentric and tiered defensive layouts.
Today, the former castle grounds have been developed as Saga Castle Park. Historical remains can still be seen throughout the grounds, which are also a popular sightseeing destination.
The predecessor of Saga Castle was Murakami Castle, which was built by the Ryuzoji clan, a powerful feudal family in Hizen Province, between the Kamakura period and the beginning of the Sengoku, or Warring States, period. After the area experienced the intense conflicts of the Sengoku period, the Nabeshima clan carried out extensive expansion and renovation during the Keicho era (1596–1615), transforming the site into the castle that became known as Saga Castle.
Saga Castle was designed with multiple layers of moats surrounding the grounds and a water-control system that served as part of its defensive strategy. Large quantities of water were drawn into the moats from the Tabuse River, allowing the surrounding area to be flooded as a way of obstructing enemy advances. This defensive arrangement gave rise to the nickname “Submerged Castle.”
Another explanation for the name is its appearance. The castle was surrounded by pine trees and camphor trees, which concealed much of the structure. From outside, the castle seemed to sink into the surrounding greenery.
During the Edo period, Saga Castle served as the residence of the Nabeshima family, and the castle town developed around it. The Nagasaki Kaido, an important highway connecting Kokura in Buzen Province with Nagasaki, passed through the town. The area consequently prospered as a post town along this route.
Moats and waterways also extended throughout the castle grounds and were used as sources of daily water. However, many of the castle buildings were destroyed by fire during the Saga Rebellion in the early Meiji period. The Shachinomon Gate and its adjoining turret survived and have been designated Important Cultural Properties of Japan.
Saga Castle was built on flat land and employed a complex defensive layout combining concentric and tiered enclosures. A moat more than 50 meters wide surrounded the castle grounds. Rather than stone walls, the outer defenses were formed mainly by earthen embankments.
Pine and camphor trees were planted along these embankments to block the view from outside. This gave the castle a distinctive appearance in which the buildings seemed to blend naturally into the surrounding landscape.
A comparison between the Keicho Otsumi Ezu, a plan showing the intended design of the castle, and later maps depicting the castle town as it actually existed reveals that some of the planned structures were reduced or simplified. The number of turrets was decreased, and the stonework around the main enclosure and second enclosure was made less elaborate.
For this reason, Saga Castle is considered to have remained technically unfinished in comparison with its original construction plan.
In 1726, during the Kyoho era, a major fire destroyed many of the castle buildings, including the main keep. Based on current historical research, the keep is believed to have been approximately the same size as, or slightly larger than, the keep of Kokura Castle.
As of 2024, plans to restore the eastern moat of Saga Castle were also underway, with efforts being made to bring the historic grounds closer to their former appearance.
In 1874, during the seventh year of the Meiji era, Saga Castle was occupied by rebel forces during the Saga Rebellion. Almost all of the castle buildings were destroyed by fire, with the exception of the Shachinomon Gate and its adjoining turret.
Bullet marks from the fighting remain on the Shachinomon Gate, providing a vivid reminder of the conflict. After the rebellion, public facilities such as schools and government offices were built on the former castle grounds, and the area developed into an administrative and economic center of Saga Prefecture.
The palace in the main enclosure, known as the Honmaru Goten, was used as a courthouse and school during the Meiji period. It was later demolished following the Saga Rebellion.
In 2004, the palace was reconstructed in wood and opened to the public as the Saga Castle Honmaru History Museum. The reconstructed building allows visitors to experience the atmosphere of the former castle palace.
One particularly noteworthy part of the palace is the Ozama, a room located at the rear of the Honmaru Goten. It was used as the private room of Naomasa Nabeshima, the lord of the Saga Domain. The room was relocated to another site in 1958 before eventually being returned to its original position.
On April 6, 2006, Saga Castle was selected as one of the “100 Famous Castles of Japan,” a list compiled by the Japan Castle Foundation.
Large-scale archaeological excavations of the castle’s main tower platform began in 2012. The investigations confirmed that the castle keep had four roof levels but five internal floors. Excavated metal fittings also provided clues about the types of windows used in the structure, helping researchers better understand the architectural techniques employed in the castle’s construction.
The former Saga Castle grounds are now maintained as Saga Castle Park. Restoration work on the eastern moat and earthen embankments has been progressing, allowing visitors to appreciate more of the castle’s historic appearance.
The reconstructed Honmaru Goten is one of the park’s main attractions. Visitors can explore the historic wooden building and learn about the history of Saga while experiencing the atmosphere of the former castle palace.
The Saga Castle Honmaru History Museum houses a wide range of materials introducing the history of Saga Castle and the Nabeshima family. Exhibitions also cover life, politics, and culture during the Edo period, giving visitors an opportunity to learn about the history of the Saga Domain.
The interior of the reconstructed Honmaru Goten faithfully recreates the appearance of the former palace. Walking through the building offers an impression of the scale and atmosphere of the castle’s main residence.
The area around Saga Castle is now home to Saga Prefectural Government offices, joint government buildings, broadcasting stations, museums, art museums, and schools. It continues to function as a center of administration and culture in Saga Prefecture.
The coexistence of historic castle remains and modern urban facilities is one of the distinctive features of the area. Visitors can explore the legacy of the former domain while also experiencing the present-day city.