Koguchi (a castle entrance) (虎口)

Koguchi is an entrance fixed to castle walls built in the medieval period and afterward, and 'koguchi' means narrow path and narrow entrance.
Other kanji characters have been used to represent Koguchi, such as '小口.'
When '虎口' is read Koko, it means a battlefield or a dangerous place in an encampment in the medieval period.


Koguchi was a front opening area fixed to castle walls or Kuruwa (walls of a castle or a space reserved for various purposes) and served as an entrance and exit for military forces of the castle. At the same time, if there was an attack on the castle, the enemy tried to come closer and closer to Koguchi, and the Koguchi became a key point in defense of the castle, so it was strictly secured and protected. A long time ago, a wooden gate was put in the open section and two turrets were built on either side near the gate for emergencies. They were simple, but in the Sengoku period (period of warring states), the Koguchi made a remarkable development and became a key factor in castle territories.

The Masugata style was developed in western Japan, and the Umadashi style (a style of defensive gateway barrier of castles) in the eastern part. As time passed and a unified authority appeared, technological integration occurred, and the Masugata and Umadashi styles both gradually spread throughout Japan. In the meantime, to ensure strict protection and security, castles equipped with two Koguchi, Masugata style inside Umadashi style, appeared.

Aizu Wakamatsu-jo Castle has of two Koguchi, Masugata style inside Umadashi style, for the north demaru (a small castle or tower built onto and projecting from a larger castle) and the west demaru. This structure helped the gate door to be in a blind spot in an attack, and in many battles of the Boshin War at the end of Edo period, they were successful in preventing the New Government Army from breaking through Kita demaru Ottemon gate, allowing the castle to be held for a long time.

Styles of Koguchi

The most basic style of Koguchi was Hirairi (with the entrance on the long side of the building), in which a Koguchi was inside a moat or earthwork. This style was intended to protect the gate from surging large armies at the front, that was a disadvantage for the castle.

In mountain castles built in the medieval period, a very steep approach was instead constructed before the Koguchi to slow the enemy's advance. This is called Saka koguchi.

To hide the inside of a castle make the front unrecognizable, a new style of Koguchi called Ichimonji koguchi was built, in which a straight-line bulwark called 'Shitomi' (a wooden lattice door that opens up vertically) was installed inside Koguchi, or another style called 'Kazashi' (walls, embankments, or plants used to prevent the enemy from seeing activities inside a castle) was installed outside Koguchi.

Kuichigai-koguchi entrance

As the scale of battles became larger, the importance of Koguchi increased, and much care was given to the design and location of moats, earthworks and stone walls to enhance the defensive capability of castle walls. In Kuichigai-koguchi entrances, earthworks or stone walls were built in different directions, not in parallel, and an entrance was constructed at the side so that the enemy was compelled to follow the S-shaped pathway. Also, the structure made enemies vulnerable to attack from the side by guns or arrows.

Masugata koguchi

At the end of the Sengoku period, Koguchi called Masugata appeared mainly in the west part of Japan. This was a square enclosure with a gate or entrance and was placed in front of a Koguchi to double protection. As a result of the construction of the two gates, even if an enemy could managed to enter the Masugata enclosure through the first gate, they were prevented from entering the castle ground by the second gate, and were vulnerable to attack in the Masugata area.

There were two styles of Masugata: Uchi-masugata (a type of walled compound, in the shape of a square measuring box, or masu, inside the entrance to a defensive installation) and Soto-masugata (a type of masugata, built outside the castle entrance). Uchi-masugata was an inner enclosure placed inside Koguchi in Kuruwa, and a gate was fixed to it to make the second gate. Upon entry into the Uchi-masugata, the enemy would be surrounded from three directions. Soto-masugata was an enclosure projected outside the main Koguchi in Kuruwa, and a gate was fixed to it to make another gate at the forefront.

In Masugatamon gate, a front gate and a rear gate were opened. The basic type seems to have been composed of Korai-mon Gate as a front gate and Yagura-mon Gate as a rear gate, but some Masugatamon gates had only a rear gate without a front gate and others had only a front gate.

Castle walls constructed in the Azuchi and Edo periods mostly had Masugata or similar types of Koguchi, and to prevent an enemy from easily proceeding straight, many of the paths following the first gate were designed to bend to the right or left.


Separate from Masugata, a Koguchi type developed in the eastern part of Japan was Umadashi. Umadashi was a small Kuruwa placed outside of Koguchi facing a moat, and was made in arc shape or U-shape by heaping up earth or constructing stone walls. Moats were constructed around the Umadashi, leaving a small part of land available for a path. A large type of Umadashi was called Umadashi kuruwa. Some Deguruwa (compounds that jut out or are slightly separated from the main components of a castle complex) and Demaru (small towers built onto and projecting from a larger castle) were made in the Umadashi kuruwa style, and the Sanada-maru at Osaka Castle is a good example.

[Original Japanese]