Fifteen Castles of Chikugo
The Fifteen Castles of Chikugo was comprised of fifteen high-ranking kokujin, or provincial landowners, based in Chikugo Province under the command of the Ōtomo clan of neighboring Bungo Province during the Sengoku period.
In Chikugo, there was no clan with the power to unify the province. In each district, kokujin maintained a state of co-existence and co-prosperity under the command of the Ōtomo, the sengoku daimyō family of Bungo who further served as the military governors of Chikugo. Among these kokujin, in particular, there were fifteen influential families known as the Fifteen Castles of Chikugo.
This group was led by the Kamachi clan, exerting their power in Chikugo from their base in Yanagawa Castle with a territory of approximately 120,000 koku. The Kamachi were split between the main branch of the clan based at Yanagawa (known as the Shimo-Kamachi) with landholdings of 12,000 chō (hectares) and a cadet family based at Yamashita Castle (known as the Kami-Kamachi) with landholdings of 8,000 chō. The Ōtomo feared that the Kamachi, as the most powerful clan in Chikugo, would acquire enough influence to become independent. To mitigate this risk, the Ōtomo gave patronage to Kamachi Chikahiro (the younger brother of Kamachi Akihisa) to form a separate family and thereby disperse the influence of the Kamachi clan.
The landowners in Chikugo led by the Kamachi clan mobilized for numerous battles under the orders of the Ōtomo. In addition to submitting to the military power of the Ōtomo, these same landowners held hidden desires to become independent of, or to oppose, the Ōtomo. The Ōtomo responded to these threats by positioning smaller gōzoku, or wealthy families, under their direct control in Chikugo known as the Taka-ikki group, enabling them to monitor the families comprising the Fifteen Castles of Chikugo.
In 1578, after the overwhelming defeat of the Ōtomo by the Shimazu at the Battle of Mimikawa, Ryūzōji Takanobu, having pacified Hizen Province, began an advance into Chikugo. The kokujin in Chikugo soon joined the Ryūzōji army. After an initial period of resistance, Kamachi Akihiro of Kami-Kamachi, Miike Shigezane of Imayama Castle, and Monjūjo Akikage, among others, later joined the Ryūzōji army.
Kamachi Shigenami of Yanagawa, the head of the Fifteen Castles of Chikugo, served in the vanguard of the Ryūzōji clan as a security officer of Ryūzōji Takanobu, but enmity grew between the two men, and Takanobu surrounded Yanagawa Castle. Following an extended siege, a tentative settlement was reached but after Takanobu discovered that Shigenami was colluding with the Shimazu clan of Satsuma, Takanobu invited him to Hizen and killed him. He then decimated the Shimo-Kamachi and took control of Yanagawa.
In the wake of the death of Takanobu at the Battle of Okitanawate in Shimabara in 1584, clans including the Kami-Kamachi, the Kuroki, the Nishimuta, the Kusano, the Hoshino, and the Monjūjo sent written pledges of allegiance to the Ryūzōji family to express their true intention not to betray the Ryūzōji. Compared to the Ryūzōji of Hizen and Saga provinces, the Ōtomo of Bungo, and the Shimazu of Satsuma, the kokujin of Chikugo did not wield as much power so they were always compelled to align with one of these clans for protection in their quest to survive the tumult of the Sengoku period.
The clans comprising the Fifteen Castles of Chikugo included: the Kamachi (split between the Shimo-Kamachi at Yanagawa Castle and the Kami-Kamachi at Yamashita Castle), the Monjūjo of Nagaiwa Castle, the Hoshino of Myōken Castle, the Kuroki of Nekoo Castle, the Kawasaki of Inuo Castle, the Kusano of Hosshin Castle, the Tanba of Mount Kōra, the Takahashi of Shimo-Takahashi Castle, the Mihara of Hongō Castle, the Nishimuta of Nishimuta Castle (and, later, Jōjima Castle), the Tajiri of Takao Castle, the Gojō of Yabeyama Castle, the Mizoguchi of Mizoguchi Castle, and the Miike of Imayama Castle.