“Tenshu”: Japan’s Castle Keeps as a Symbol of Powerful Leaders

To Merely Gaze Upon Azuchi Castle Brings Joy

The tenshu or keep is the largest yagura (turret) in the honmaru (main compound) of a Japanese castle. Yagura were used as reconnaissance points to look out for enemy movement. They also played a defensive role as archers could fire arrows from them to keep invaders away.

Although Gifu Castle, as of 1569, had a large yagura, it was not until 1576, when Oda Nobunaga constructed Azuchi Castle that the main turret in a castle began being referred to as a tenshu. Originally, the name was written using the Chinese characters 天主 (tenshu) meaning “master of the realm,” but this was later changed to 天守 (also tenshu), literally “protector of the realm.”

Historical records at the time mention that the main turret at Azuchi Castle had six floors above ground and one floor below. The very top floor was in the shape of a square, measuring three ken (around 5.45 meters) in length on each side. Surrounding the top story was a type of veranda known as a mawarien with a kōran decorative balustrade and handrails to prevent falls. It is said that this upper floor was Oda Nobunaga’s living space and that all the floors were in the shoinzukuri residential architectural style that appeared during the late Muromachi period (1333－1568).

The Jesuit missionary Luís Fróis described the tenshu at Azuchi Castle in his book History of Japan, saying that it was a more majestic and refined edifice than the towers of Europe, and to see it even from afar brings great joy and contentment.

These words give a glimpse into Nobunaga’s reason for constructing the keep. Namely that castles, which up to then had been built as defensive structures for war, became an architectural demonstration of Nobunaga’s authority as tenkabito or “ruler of the realm.” Azuchi Castle was regarded as a showpiece, a visible display of Nobunaga’s power as he came closer to conquering the Kinai region, the ancient provinces around the former capital of Kyoto.

In 1582, Akechi Mitsuhide, after betraying and overcoming Nobunaga in the Honnōji Incident, temporarily took over the castle, but immediately lost in battle to Toyotomi Hideyoshi. Azuchi Castle was burned down. Some parts of the structure still remain, including the foundation stone at the base of the tenshu, and excavations have also uncovered tiles decorated with gold leaf.

This is thought to be the oldest existing illustration that depicts the remains of Azuchi Castle and appears in Nihon kojō ezu gōshū azuchi kojō-zu, a pictorial map of old Japanese castles created in the mid-to-late Edo period (1603–1868). (Courtesy National Diet Library)

Continuation of Tenshu Construction under Hideyoshi and Ieyasu

The next keep to appear was built by Toyotomi Hideyoshi. This was at Osaka Castle. He ordered the daimyō under his command to mobilize 50,000 people to build the tenshu and construction was completed in around 1585, three years after Nobunaga’s death.

From various artwork, including the Osaka natsu no jin zu byōbu folding screens depicting the 1615 summer campaign of the Siege of Osaka, the tenshu is presumed to have been in the form of an apparently five-tiered lookout tower that actually had seven stories. The bottom two floors of this keep were decorated with irimoyazukuri hip-and-gable triangular roofs and above that was a turret-style building that allowed views of the surrounding area.

The exterior was graced with gold and black, and the walls layered with black lacquer would have shone like fine lacquerware. It had decorative metal fittings, gold leaf tiles, and other elaborately carved tiles too. Gold was Hideyoshi’s signature color, so it was very much built in his style. However, in 1615, the castle fell to Tokugawa Ieyasu during the summer campaign of the Siege of Osaka.

In 1620, the Tokugawa shogunate began planning the rebuilding of Osaka Castle. The reconstruction was completed in 1629, but all symbols of Hideyoshi’s authority were destroyed and buried beneath mounds of earth. The stone walls were also completely newly built with none of the old materials reused. With this, the Toyotomi clan tenshu was no more. It was only during excavation work in 1984 that a part of the wall from Hideyoshi’s time was discovered.

Meanwhile, a tenshu was also built at Edo Castle, which served as the seat of government for the Tokugawa shogunate. It was estimated to be 68 meters in height. This was 10 meters taller than the keep at Osaka, which was thought to have stood at 58 meters. There are various theories regarding its height, one of which was that it symbolized Edo (now Tokyo) surpassing Osaka. The tenshu was a representation of government and therefore had to express dominance.

The exterior was also given a different appearance so that it was apparent power had shifted from Toyotomi to Tokugawa. The keep during the time of Ieyasu, the first Tokugawa shōgun, was covered with white plaster, a clear differentiation to the gold and black of Toyotomi’s tenshu at Osaka Castle.

In addition, because the Tokugawa shogunate’s authority was hereditary, the castle was rebuilt under each new shōgun. While Ieyasu’s white-plaster tenshu was connected to the yagura, when the second Tokugawa shōgun Hidetada came to power, the whole structure was demolished and rebuilt. The new tenshu was also covered in white plaster, but it stood independently.

Edojō go-honmaru go-tenshukaku tatekata no zu, a structural plan of the honmaru tenshu at Edo Castle which was completed in 1638 under the third Tokugawa shōgun Iemitsu. (Courtesy the Tokyo Metropolitan Central Library Special Collection Room)

The third Tokugawa shōgun Iemitsu once again demolished the castle and had it rebuilt. This time the hafu triangular-shaped gables attached to the edges of the roof, a feature of Japanese architecture, were clad in copper. The walls were also copper-plated and the roof tiles made of copper too, which made them sturdier than if covered only with plaster.

Iemitsu died on April 20, 1651, and, as was custom, the tenshu was again set to be rebuilt. However, in 1657, disaster struck Edo. This was the Great Fire of Meireki, in which 100,000 people are thought to have died. The keep was lost in the fire and never reconstructed. The ruins of the Edo Castle tenshu foundation that can still be seen in Tokyo today are what remains of Iemitsu’s construction.

Passing as Tenshu

As mentioned above, it was Nobunaga who first used the term tenshu (“master of the realm”), and the continued construction by Nobunaga, Hideyoshi, and the Tokugawa clan meant this main turret still carried the symbolism of tenkabito or having ruling authority. It is a fact that after the Tokugawa clan took control of the whole of Japan, powerful daimyō like Date Masamune ceased building tenshu due to this meaning.

Moreover, in 1615, the shogunate issued strict edicts to control the daimyō. This included prohibiting them from building new castles and requiring them to obtain permission from the shogunate prior to carrying out repairs on existing castle structures. This basically meant that they could not build new tenshu, unless as part of reconstruction.

There are currently 12 castles in Japan that still have their original tenshu; ones that were largely constructed prior to the 1615 edicts or that underwent repairs after that time.

That said, there were times when sanjū yagura or three-story turrets were constructed as the main turret and seen unofficially as keeps. This is the case for the one at Hirosaki Castle in Aomori Prefecture, one of the 12 keeps mentioned above. Newly constructed in 1810, on paper it is referred to as a sanjū yagura, or more officially a gosankai-yagura, a term also meaning a three-story turret. It is the same for the tenshu at Marugame Castle in Kagawa Prefecture.

The sanjū yagura at Hirosaki Castle, unofficially recognized by the Tokugawa shogunate. (© Pixta)

There was a tenshu at Kanazawa Castle, the seat of the Maeda clan in Kaga Domain (now Ishikawa Prefecture), but after it burned down in 1602, a sanjū yagura was constructed instead on the keep’s base. This new one was also lost to fire and after that, most likely in deference to the Tokugawa shogunate, it was not rebuilt.

These days when people think of Japanese castles, it is the image of the tenshu that rises most prominently in their minds; this was originally a symbol of the authority held by Nobunaga, Hideyoshi, and then Ieyasu and the Tokugawa shogunate.

(Translated from Japanese. Banner photo: Osaka Castle constructed by Toyotomi Hideyoshi, as depicted in the Osaka natsu no jin zu byōbu folding screens. Courtesy Osaka Castle Museum.)