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Warning! This page contains spoilers for the Emperor of Nihon-Ja.
Ran-Koshi is a largely natural fortress in northern Nihon-Ja, made of high mountains. It appears in The Emperor of Nihon-Ja, when Shigeru needs somewhere to weather out the winter, Ran-Koshi proved ideal as it was defensible and located in an ideal location. It was regarded as a fable by most.
It used to contain a fortress of wood protecting the entrance into the valley and a small "village" that could easily house an army, as it does in the Emperor of Nihon-Ja.
There is an exit and a watchpoint which looks out over the mountains and passage leading into the valley, both aren't easily accessible or found but in the book they are used to mount an attack against Arisaka's army by Will and his friends.The sides of the fort are natural massive cliffs that are impassable by any army.
The Kikori rebuild the main man-made fortress which protects the entrance into the valley before re-building the houses in the "village" located in the main part of the valley.
There was also a hidden path which led out of the fortress, for if the defenders had to retreat.
Sometime in the past Ran-Koshi had been used by bandits of some sort but they had long since deserted the place. It is then used by Shigeru and his men when they are sheltering from Arisaka during the winter.