Decree of One Castle Per Province (一国一城令)
The decree of one castle per province was established by the Edo bakufu (Japanese feudal government headed by a shogun) on August 7, 1615. This decree stipulated disposal of all castles except one castle for feudal lord's residence or government office in one province (the province here means not only a province but also feudal lord's territory [subsequent domain]).
When a province was divided to be owned by several feudal lords, each lord was allowed to have one castle (for example, Ozu-jo Castle, Matsuyama-jo Castle and Uwajima-jo Castle in Iyo Province, etc.), and when a clan of feudal lord owned territory across a few provinces, they were allowed to have one in one province (for example, Tsu-jo Castle [Ise Province] and Ueno-jo Castle [Iga Province] of the Tsu Domain).
This decree was not implemented uniformly but practiced flexibly very much, and when it came to a large scale domain, families or vassals of high rank or income were the substantial castle owners and, in fact, there were some examples that some of them kept several castles.
According to the book 'Journal of House Sitting in Edo' written by Hirofumi YAMAMOTO and other materials, the Mori clan reported to the bakufu after the clan destroyed castles such as Iwakuni-jo Castle while leaving Hagi-jo Castle in Nagato Province, then, the bakufu replied, 'Since the Mori clan owns only two provinces, Suo and Nagato, you did not have to destroy Iwakuni-jo Castle in Suo Province, additionally.'
It is believed that, coupled with humbleness to the bakufu and calculation on the branch domain's regulation within the clan, the castle was destroyed inadvertently.
Additionally, castles such as Shiraishi-jo Castle in the Sendai Domain and Yatsushiro-jo Castle in the Kumamoto Domain owned by the descendants of those who particularly contributed to the bakufu among vassals of feudal lords were exception to disposal of castle. In addition, the Sendai Domain also kept many actual castles by calling them fortresses.
In the Akita Domain, Odate-jo Castle and Yokote-jo Castle (both were burnt down in the Boshin War) were exception to disposal, as a result, the domain was allowed to have three castles in total, which were the above-mentioned two castles along with Kubota-jo Castle. Also, some say that in return for the grand feudal lord Satake clan's change of the territory to Akita, they were allowed to own castles in the districts that had been politically unstable.
Significance of enforcement
As a result, the number of castles which were said nearly as many as 3,000 in the Azuchi Momoyama period dropped sharply to about 170 (about 300 when regional government offices included), and consequently, grouped housing of vassals and domain people in castle towns progressed more.
Its purpose was to strengthen the Tokugawa family's domination of the entire country by regulating the power (military force) of the feudal lords; it was focused in particular on the western provinces, where there were a lot of nonhereditary feudal lords.