Defensive castle entrance, or yin entrance. A generic term for various types castle entrance *koguchi 虎口, considered to be defensive in character. These were designed to make it very difficult for enemy forced to invade the castle, but did not permit easy exit or outgoing sorties. They usually have large or complex barriers *umadashi 馬出 protecting the entrance.
Belonging to this category of entrances are Edo period designs, for example the spiral entrance tomoe koguchi 巴虎口 where a series of gates are connected by walls twisting round in a spiral formation, or the right-angled barrier kane no umadashi 曲尺の馬出 where the entrance is located in an indented corner of the castle, with a square barrier preventing direct enemy access. Medieval mountain castles and border castles, built with the expectation of having to fight superior numbers of enemy troops with an inferior numbered force, utilized a defensive entrance. The term in no koguchi is used in contrast with *yō no koguchi 陽の虎口, an offensive entrance.