In the late 1500s, Toyotomi Hideyoshi was the strongest ruler in Japan and he was trying to tie up the last areas that weren’t under his rule.
Nakatsu was one of these areas. In order to subjugate the island of Kyushu, he sent his top strategist Kuroda Kanbe. There were several battles and the area was more or less taken care of, but in order to cement control, a castle was needed.
The location selected was along the Yamakuni river at the coast in Nakatsu. Many castles are in the mountains so that Hideyoshi’s army can take advantage of the natural terrain. However, in a flat flood plain area, Kanbe needed to add elements to make the castle stronger. So he put it directly on the river and built up several layers of walls. Along with the walls, motes were dug.
Even the town was rearranged to make it harder to attack. The main road to the castle was offset in order to make a choke point to delay troops. All of the temples were put together in order to provide a barrier of large buildings to supply another layer of wall-like defense. An added feature was that the motes around the castle were made to be too shallow to swim and too muddy to walk through. It never was attacked, so it seems to have done the job.
Currently, the walls under the castle are original and fragments of outer walls can be found scattered in the surrounding areas. There are no good images of the original castle, probably due to tight security. It was burned down during the 1873 riots during the Meiji period when Japan transitioned from a closed medieval to an industrial nation. Unfortunately, the castle was never photographed during this period.
The current building was erected in 1964 by the descendants of the former rulers and occupants of the castle. The model for the castle was Hagi castle that built a few years after Nakatsu castle and was burned down in 1874.
See the link below for a look at an old photograph of it.
The Hagi castle is in nearby Yamaguchi prefecture.
The castle is a museum of items from the last ruling samurai family, the Okudaira, which has several historic items that date back to the Sengoku period. The Okudaira family took part in the battle at Nagashino castle, causing the launch of the successful unification of Japan by Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu. One of the rooms is dedicated to the role that the Okudaira played, and from this role, they were able to gain a degree of prominence with the Shogun and become a Daimyo (local ruling) clan.
Nakatsu has adopted the castle as the symbol of the town. Many of the town’s manhole covers are decorated with a design of the castle.