Tenshu

What a tenshu is

A tenshu is the tallest tower at the centre of a castle. It served for observation and command, and as the building that displayed the lord's authority, which is where the design effort went.

It does not always stand alone. At Matsuyama Castle in Ehime Prefecture the keep, a smaller keep and two corner turrets are linked by Tamon-yagura around a courtyard, and records name the group a connected keep. The small keep is described as a two-tier turret at the south-east angle of that square arrangement, the south corner turret as its counterpart at the south-west.

The parts are designed as one. The small keep has walls of Shikkui with Shitami-itabari at the base and the eaves soffit plastered over; the entrance corridor turret is described as continuous with the north corner turret in the same treatment. The word for continuous marks the group as a single composition.

A smaller keep is the tower next in size to the main one. The Matsuyama entry calls it second only to the keep and part of the imposing core of the group.

Reconstructed keeps

Some keeps survive from the Edo period and some were rebuilt later, and the two fall under different protection. The small keep, south corner turret and corridor turrets at Matsuyama were reconstructed in 1967 and are recorded as a Registered Tangible Cultural Property. Designation as an Important Cultural Property applies to surviving buildings.

Others were rebuilt in reinforced concrete. The keep at Osaka Castle, built in 1931, is recorded as eight storeys of steel-reinforced concrete covering 1,199 square metres, and is described as built as a museum when the former inner bailey, until then army land, was made into a park, funded by public subscription and long familiar as a symbol of the city. It takes the form of a keep but was planned from the start as a museum.

When writing about one, say whether it survives or was rebuilt, and if rebuilt, whether in timber or concrete. The category of protection and the date of construction both appear in the record.