The nawabari of Himeji castle.
Nawabari translating directly into English as `stretched rope', is the process of site planning for the castle. It is the most important step in the castle building process. Many warlords believed that the nawabari determined the fate of the castle. The nawabari of Himeji-jo consisted of sudden descents and switchbacks. This was so that the enemy would become slow and bewildered, and find it impossible to locate the entrance to the main donjon. ( Japan: An Illustrated Encyclopaedia p166 )



The Japanese word tenshu is often translated to mean 'donjon'. It refers to the main tower of the castle. This tower has the most powerful, symbolic role. The early tenshu were simple observation towers that were built on top of residential buildings or on the roof of other towers. They were used for observation, confining prisoners, protecting water supplies and as a post for the castle lord in times of siege.

A change in the tenshu was bought about by the building of Azuchi-jo in 1576. The tenshu became a symbol of the owners power and influence. As the castle lost its function as purely for military purposes and adopted a more political role, the style of the tenshu became more symbolic. The tenshu became a structure of increasing detail and beauty. In the succeeding years after Azuchi-jo was built castles such as Osaka-jo, and Fushimi-jo carried on building castles with an emphasis on the tenshu. The tenshus ranged from simple donjons like that of Hikone and Maruoka castle, which had a single tower, to donjons of great complexity like that of Himeji-jo which had three minor keeps in addition to the Great Tenshu. ( Hinago Japanese Castles p104)

The same techniques that had been previously used to construct pagodas in Japan were employed for the construction of the tenshu. A pillar that reached up from the foundation podium through each story to the ridge was inserted. This stabilised the frame of the central structure. The builders added decorations such as gables, on the roof, outer walls and windows of the tenshu, to emphasise the height of the structure and create a look of power.



Ranseki-zumi, at Matsumoto castle.
(Japanese Castles p86)
Unlike in Europe the use of mortar was not practical in Japan, due to the high incidence of earthquakes. Japanese castle walls were built of stone with builders using a special dry masonry technique to lock rocks into position.

The simplest form of ishigaki (stone wall building) was used for the early fortifications that were built on top of mountains. There were great difficulties in building sophisticated and large scaled walls on top of mountains, and a technique called ranseki-zumi (disordered stone piling) was used. Large rocks were piled up to make a wall and small stones were inserted into the gaps to lock the rocks into place. Any gap was filled in so that attackers could not use them for footholds when trying to scale the wall.( Hinago Japanese Castles p84)

The curving walls of Osaka-jo.
( From Castle to Teahouse p6.)
Azuchi-jo was one of the first castles to be of the mountain-plane type, and it bought about a change in wall building. The difficulties presented in building the walls of mountain top castles were no longer present. There came a new need due to the introduction of firearms to build a higher and stronger wall, and a technique called gobo-zumi was introduced. In this method large rocks were embedded or built up against an existing hillside or a prepared mound. Small stones were then inserted locking the wall together. Both the ranseki-zumi and the gobo-zumi systems allowed for flexibility and movement during an earthquake.( Hinago p84) Many of the walls constructed using these two techniques are still standing today, 400 years after construction.

The curvature of the walls also varied according to the site and structure of the castle. Walls were steep where the ground was firm and the weight of the castle to be supported was light. Where the ground was not firm and the castle to be supported was of great weight, the walls spread their curves. Curvature of the walls was also necessary for defence against earthquakes.

Kenchi-zumi is the name used for the method of reconstructing castle walls today. Square joints are assembled with neatly matching joint faces.( Hinago p84)