This town is organised around an administrative seat rather than a castle. Kōjiro was the jin'ya town of the Nabeshima, and its samurai residences — enclosed by stone bases and clipped hedges — stand along a regular grid of streets. A jin'ya is the office of a lord who held no castle, so the scale is smaller, but the thinking behind the layout is identical to that of a castle town.

The garden at the Nabeshima residence is open, and it shows the point that plans rarely convey: the house and the garden were designed as one continuous thing rather than as a building with planting around it. Kōjiro was designated an Important Preservation District in July 2005, and it is one of the few jin'ya towns on the national list. The Shimabara railway station is close by, Isahaya is about thirty minutes away by car, and this stretch of the Ariake coast is mild enough that the hedges stay green through the winter.

Even without a castle, the divisions of rank survive in the ground plan. The consistency of the wall heights, and of the way the hedges are cut from one house to the next, is not aesthetic agreement between neighbours; it is what happens when rules about status extend to the outside of a building as well as to the inside of it. And with no castle dominating the view, those rules are easier to read here than they are in a larger town. With nothing else dominating the view, the hierarchy itself becomes the structure on display, and it is remarkably legible for something that was never intended to be looked at as a composition. A castle would have distracted from it entirely.