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.
虎口の内側に「蔀（しとみ）」や、虎口の外側に「芎（かざし）」と呼ばれる一文字形（一直線）の防塁（土塁や石垣など）を構築し、前面から郭内が容易に視認できないようにしたものも現れ、一文字虎口（いちもんじこぐち）と呼ばれた。