Tenshu

The main keep or tower of a castle is called the tenshu. The tenshu, also called the donjon, is by far the most important part of the castle. Located at the center of a castle's defenses, it was the safest place to be during an attack. In times of war it was used as a lookout tower, as a prison, as protection for the water supply, and most importantly as a military command post. As the turbulent conditions in feudal Japan calmed down, the tenshu's function became more and more symbolic. Tenshu eventually came to be seen as a symbol of a daimyo's strength and prestige rather than as a military structure. These later-built tenshu are ornately decorated and are considered by many to be some of the most beautiful buildings in the world.

The earliest tenshu were simply large houses with belvederes (small towers) built on the roofs. This design later grew to become the multi-leveled towers found on castles such as Hikone and Maruoka. This one-tower design gradually evolved into elaborate designs such as the one found at Himeji Castle.

Tenshu were constructed atop ishigaki to give them a high and firm base. They were constructed similarly to pagodas: two or more strong wooden poles were planted vertically in the foundation, then the rest of the structure was built around the poles. Unlike their counterparts in Europe, Japanese castles are not built from stone but rather from timber and clay. These materials, as well as the ishigaki base, are ideally suited for lasting through the earthquakes which frequently hit Japan. The tradeoff for this flexibility is that the wooden walls are more vulnerable to fire, lightning, and artillery. Plaster and lacquer were used to fireproof the outer walls, but many tenshu were still lost to internal fires, lightning strikes, and bombings.

As the functional heart and brain of the castle, the tenshu was heavily guarded. Tenshu are rarely found alone; they are usually flanked and surrounded by yagura, walls, moats, and/or gates. This, of course, was in addition to the defenses of the outer maru. In the unlikely event that any invaders managed to break all the way through, the tenshu itself had several defensive features:

On the corners and along the sides of most tenshu [and yagura] you can see small protrusions like the one pictured here. Their name, ishi-otoshi, translates to "stone dropping"--and that is exactly what they were used for. As an attacker would try to climb up the side of the tenshu, defenders inside the tenshu could drop heavy rocks, boiling water, or even hot oil through the openings, sending the would-be invader to his doom.

The exterior design of the tenshu is also a form of defense. By complicting the facade with elaborate designs and features, the designers of a castle could make it hard for an invader to calculate how many floors were actually inside. Even in seemingly simple-looking tenshu, there are often extra floors on the inside that can not be discerned from the outside.