Honmaru is a term used to refer to a kuruwa (castle compartment) at the center of a Japanese castle.
They are also called Ichinokuruwa, Honkuruwa, etc. For castles built in the middle ages, it is said that they were called Honjo, Mijo, Tsumenomaru, etc., depending on the castle. In studies on castles built during and before the early modern period, they are sometimes called Shukaku. Accordingly, the term Honmaru is sometimes used to refer to the essence of matters, the core of an organization, etc.
In some castles, the honmaru was used as a living or administration quarters for the castle owner, by building gorgeous buildings such as a honmaru-goten. In other castles, such functions were provided in a Ninomaru (second central castle compartment) or a Sannomaru (third central castle compartment), and the honmaru was a small area which functioned as a tsumenomaru during battle. Tenshus (tower-like structures) were also starting to be built in later ages. A small kuruwa with a tenshu inside or outside the honmaru is sometimes called Tenshu-kuruwa or Tenshumaru.
Some honmarus were built based on the physiognomy of a house. It is said that according to the physiognomy of a house, Ote-mon (main gate) should be located to the south-east of the center of the castle (honmaru or goten), and a corner located to the north-east (direction of demon's gate) of the center of the castle should be removed by carrying out Sumikaki (corner cutting) in order to avoid the influence of the demon. It was believed that the tenshu or gosangaiyagura should be built to the north-west of the center of the castle.
Very few honmaru-gotens have survived, and the only complete one can be found in Kochi castle. Main entrance portions can be found in Kawagoe castle and Matsumae castle. Many reconstruction projects of tenshu or yagura are being considered. However, very few of them have been realized, due to shortage of historical records and lack of cultural impact.