Reigandō is a cave in the hills west of Kumamoto, on the grounds of the temple 雲巌禅寺 Unganzenji. Musashi withdrew there in about 1643 and wrote the five scrolls over roughly two years, finishing in the fifth month of 1645.
It is not large. It is stone, and damp, and there is a view out. The approach is lined with the 五百羅漢 — five hundred stone rakan figures, carved in the eighteenth century by a merchant of Kumamoto over some twenty years, each one different, most of them now worn and lichened. They were not there when Musashi was.
Getting there. Unganzenji is about twelve kilometres west of Kumamoto city, reachable by bus from the Kōtsū Centre. It is open daily and there is a small admission. The walk from the temple to the cave takes a few minutes and is uneven.
What to correct before you go. The standard telling makes the cave renunciation — the old warrior shedding the world. He was a stipended retainer of one of the largest domains in Japan, living on Hosokawa rice, on temple land, with people bringing him things. He was also ill, and knew it. The withdrawal is at least as much a dying man arranging his last work as it is a sage seeking clarity.
And the thing worth carrying in. What the cave produced was not the doctrine — most of the technical content already existed, in the Thirty-Five Articles he had written for Tadatoshi four years earlier. What the cave produced was the architecture: the decision to divide it into five, to name them Earth, Water, Fire, Wind and Void, and to order them foundation before flow before application. That decision is the only reason the book lasted, and it is the one thing in it that could not have been written in 1641.