Shijo refers to an outer citadel, fort or fortification built to protect the main castle and perform a backup role. It was called an 'edajiro' (branch castle) to distinguish it from 'nejiro' (root castle) which indicates the main castle. There are many types depending on the role and they also varied in scale. In the Sengoku Period (Period of Warring States) (Japan) a diverse network called 'shijo-mo' was created between the main castle and shijo.
Types of Shijo
An outer citadel: Used for defending the main castle. Mostly built near the main castle and in strategic locations such as mountains suited to defense.
Directly Controlled Shijo
Castle Built for Attacking Enemy Castle
A facing castle, confronting castle or annex castle: A castle built to attack the enemy castle
(Sometimes also indicates a castle ancillary to main castle)
Indirectly Controlled Shijo
Border castles or boundary castles: Castles on the perimeter of the territory
Banjiro or bantejiro (both mean guard castles): A branch castle without its own lord, directly controlled by the main castle (Osaka-jo Castle and Nijo-jo Castle in the Edo period are among such castles of the shogunate government). Linking castle or communications castle: A castle playing the role of garrison, for contact and communication via smoke signals, for checking travelers crossing the border, and for gathering, transferring or accommodating soldiers.
Battle castle: Simple castle built as a temporary structure during field battles.
Shijo-mo (branch castle network) refers to a defense or governing network of castles, forts and fortifications called branch castles (shijo, edajiro, hajiro) performing a support function and scattered around a main castle which is the stronghold or base in each region developed mainly in the Sengoku Period (Japan).
The scale varied and for example Minowa-jo Castle boasted a shijo-mo consisting of three hundred and two shijo and forts large and small.