Yamajiro (mountain castles) (山城)
Yamajiro refers to castles built on steep mountains. In Japan, Yamajiro was one of the castle classification methods determined by Edo period military strategists. In medieval Europe, castles built on mountains were called as Hohenburg.
The construction of castles on mountains was a universal practice as high ground is beneficial from a military defensive standpoint. Except for Japan, many castles were built on mountains in Three Kingdoms period (of Korean history), from which the castles of ancient Japan are thought to have been modeled, and in medieval western Europe. The Port Arthur stronghold and the Maginot Line also consisted of fortifications built on mountains.
Although steep mountains are desirable to construct fortifications that are expected to be built on places effective for defence, they are difficult to live; thus, fortifications were established distant from the settlements that they served to defend. As a result, many were constructed to serve as purely defensive installations. In peacetime, the lord of the castle would often live with other people at the foot of the mountain while they moved to the castle on the mountain to barricade themselves in it when their enemies attacked them.
Japanese mountain castles include those of Korean style, which built from the Kinki region to the north of Kyushu from the Asuka period to the Nara period, those built in the late Sengoku period (period of warring states) and those built in the early modern period, from the latter part of the Azuchi-Momoyama period to the Edo period. Of the mentioned above, the mountain casltes of Korean style are discussed in the section on ancient mountain casltes.
When a medieval mountain castle was constructed, the castle itself was built on the top of a mountain and their residence at the foot of it. The castle on the mountain top served mainly as a defensive facility and the people there usually lived in the residence at the mountain's base. The castle at the peak was not meant for long-term habitation; consisting of structures with hottate bashira (earthfast posts), simple turrets and fences.
Regarding small castles, a simple building was made to store food and weapons and fences, moats and mud walls for defence were made by using natural terrain, whereas medium-size castles had Kuruwa (walls of a castle), a main compound on one peak and a secondary compound on another peak, and residential facilities that made it possible to hold out against a long-lasting siege. Large castles had subsidiary castles on surrounding mountains, making the entire mountain range serve as a fortification. Usually moats were made deep just enough for a spear to reach the bottom of it. That was due to a greater defensive benefit, which meant that castle defenders were able to stab with spears attacking soldiers who fell into the moat.
During the Warring States period, permanent structures with foundations were built and able to accommodate long-term inhabitants. Typical examples of these include Tsutsuji-ga-saki Castle (Yamanashi Prefecture) of the Takeda clan in the Kai Province and Ichijodani Asakura Clan Ruins (Fukui Prefecture) in the Echizen Province; Ichijodani Castle had a residence and their castle town within the valley and defend themselves making the castle on the peak of the mountain as their base when enemies attacked them.
Compared with hirajiro (flatland castles) and hirayamajiro (low mountain castles), yamajiro (mountain castles) tended to be smaller and some of them did not develop into cities and have been preserved as whole ruins of large castles, including the Gassantoda Castle, a mountain castle in the Sengoku (civil war) period, and Takeda Castle (Hyogo Prefecture), Takatori Castle and Oka Castle (Oita Prefecture) in the early modern period. These serve as both historical research sources and tourist attractions.
The first defensive military facilities to be built on mountains in Japan were Yayoi period highland settlements. From the Asuka to the Nara periods, invasions from the Tang Dynasty China and the Korean Silla Kingdom led to the construction of ancient mountain castles (Korean style mountain castles) throughout Western Japan.
In the Middle ages, anti-Bakufu forces led by Emperor Godaigo started to build castles on mountains by in order to resist the power of the Bakufu from the latter Kamakura period to the period of the Northern and Southern Courts (Japan). It is thought that early examples of them were Masashige KUSUNOKI's Chihaya Castle and Akasaka Castle as well as Kontaiji Castle, which used the mountain temple "Kontai-ji." The Southern Court later imitated these and constructed mountain castles in various locations. It became common that warriors constructed their residence on the flatlands at the foot of a mountain and their castle on the mountain behind the residence, and barricade themselves in the castle when the battle started.
As war became continuous during the the Sengoku period, permanent facilities that allowed inhabitants to withstand long-term battls were built. In the latter Sengoku period, a lord's residence was constructed within the main walls of their castle at the top of the mountain, while the lord made his vassals and their clans live on the mountainsideas as his hostages. From the latter Azuchi-Momoyama period to the early Edo period, they started to establish castles with stone walls and that type of castle was also used for mountain castles. It was after the ikkoku-ichijo (lit. one castle per province) order that hirayamajiro (low mountain castles) and hirajiro (flatland castles) became the most common type of castles in Japan.
The Shift from Mountains to Flatland
In the latter Sengoku period, Hirayamajiro (castles built on a hill in a plain) and Hirajiro (castles built on flatland) with their castle towns became the mainstream. There are several reasons for this and they will be mentioned below.
The Normalization of Conflict
It became inconvenient to barricade themselves in a mountain castle only when enemies attacked them; it was necessary to stay there in preparation for battles. For this reason, residential facilities were constructed at mountain castles but it was inconvenient to travel back and forth between the foot of the mountain and the mountain castle.
Established Control of the Sengoku Period Daimyo
It was inconvenien in terms of transportation to construct a castle on the top of a mountain in order to strengthen the Daimyo's control over their territory. In addition, it became necessary to display castles as a symbol of a daimyo's power so they were constructed not on steep mountains but where they could be seen by all.
Introduction of Firearms
Castles defended by wooden fences and shallow moats were vulnerable to attack by firearms. For this reason, castles came to be defended by numerous deep moats and walls. Castles were also designed to allow defenders to use firearms to fire on attackers, so there was no longer the need for the depth of moats to be restricted to the length of a spear. Therefore, it was more effective in Hirayamajiro Castle or Hirajiro Castle to make deep moats.
Increased Size of Castles
As Sengoku Daimyo (feudal lords) established their sovereignty, they made their Kokujin Ryoshu (local samurai lords) in various provinces live within the castle towns in order to ensure that the latter was completely obedient to the former. It was therefore necessary for castles to increase in size, but mountain castles had size limitations.
As a mountain castle was changed into a flatland castle or a low mountain castle, a new main castle was constructed at the foot of a mountains and a former mountain castle was used as a Tsume-no-shiro (a castle that guards the back of the main castle) (such as Hagi Castle) or a castle site was moved to a low hill or flatland (such as Fukuyama Castle (Bingo Province)). Like Odawara Castle, as a result of the repeated expansion of a castle that was originally a mountain castle and castle town at the foot of the mountain, both merged and developed into a large flatland castle surrounding its entire castle town by the sogamae (outer citadel).
However, it was not possible to replace all mountain castles with such expanded flatland castles and low mountain castles. These remained the residential castles of the daimyo themselves, and the mountain castles throughout the country remained strong. In addition, some minor daimyo had to stay in their traditional mountain castles, unable to move to a flatland castle or low mountain castle. Among these mountain castles are those that developed differently from previous mountain castles in order to increase defensive capability against firearms. For example, radial moats were often used in the western region. Some mountain castles stopped using their traditional wooden fences and started to use the architectural styles of flatland castles and low mountain castles, surrounded by mud walls with portholes, and became an impregnable fortification as if it were a pillbox (such as Takatori Castle).
It was after the Edo period when most castles became flatland castles or low mountain castles due to the Ikkoku Ichijo Rei (Law of One Castle per Province) that ordered to destroy mountain castles throughout the country. However, many of the Daimyo's residential castles of the Edo period inherited the medieval style, which combined the residence at the foot of the mountain and the mountain castle behind the residence to barricade themselves in case they had a battle, including Iwakuni Castle, Hagi Castle and Kagoshima Castle. There are also examples such as Sendai Castle which were constructed as mountain castles during the Edo period but later went on to expand and undergo the transition to a low mountain castle.