Hira-jiro castle (平城)
A Hira-jiro castle indicates a castle built on flat land. It is a castle category based on geographical features that was classified by military scholars in the Edo period.
Before and during the Sengoku period (Period of Warring States in Japan), most castles were built on mountains (these were known as Yama-shiro castles); very few were built on flat land. However, if no suitable mountain could be found to build a castle on, one was built on flat land. In such cases, one or more rivers were used as a line of defense for the castle, with mounds and walls built for its protection.
Nagoya-jo Castle, Sunpu-jo Castel, Ni-jo Castle and Hiroshima-jo Castles are typical examples of Hira-jiro castle in early-modern times. Although Edo-jo Castle and Osaka-jo Castle are sometimes considered Hira-jiro castles, they are sometimes classified as Hirayama-jiro castles (those built on a hill in a plain). Examples of Mizu-shiro (castles built on lakes or marshes for defensive reasons) include Takamatsu-jo Castle (in Sanuki province), Imabari-jo Castle, Nakatsu-jo Castle, Takashima-jo Castle, and Zeze-jo Castle (the first three examples given here all appear in the list of the top three castles in Japan).
Hira-jiro castles were based on the following two types of buildings: Hokei-yakata buildings, which were samurai residences built in the early Kamakura period and the period of the Northern and Southern Courts (Japan) on flat land surrounded by mounds and walls; and Ta-te buildings, such as the Shugosho (provincial administration offices) where shugo (military governors) lived in the later days of the Muromachi period and the Sengoku period. Shugosho were built to imitate the gosho (residence) of the Muromachi bakufu (Japanese feudal government headed by a shogun), and Yama-shiro castles were mostly used as a tsume—no-shiro castle (a kind of castle of last refuge). Shugosho were built at a strategic spot along a main road or water transportation system.
Local lords came to use shugosho more frequently, and castles were built in a similar style. The Muromachi-dai, which Nobunaga ODA built as a residence for Yoshiaki ASHIKAGA in the late Muromachi period, and Ni-jo Castle are believed to have influenced the architecture of Hira-jiro castles in the later part of early modern times.
In early modern times, castles with a tenshu (main tower) were built. However, the building of tenshu gradually became less common for the following reasons: the building of tenshu without permission was prohibited following the enactment of Buke shohatto (Laws for the Military Houses); the building of tenshu became unnecessary; some tenshu were destroyed in natural disasters; and the construction of tenshu put too much of a burden on a domain's finances.