The history of the castle prior to the conflict between the Tokugawa and Takeda clans is rather unclear. According to one version, the castle was built in 1416, when Imagawa Sadayoshi (1325–1420) was governor of Suruga Province and half of Totomi Province. Allegedly, it was he who ordered Imagawa Norimasa (1364–1433) to build this fortification. However, no reliable evidence has been found to support this. Another version is considered more plausible, according to which the castle was built during the conquest of Totomi Province at the end of the 15th century by Imagawa Ujitsuna (1473–1526) and his general Ise Shinkuro (Hojo Soon). In this case, another of Ujitsuna's generals, Kusima Masashige (1492–1521), is considered responsible for the construction.
In 1556, the Shimazu clan launched a campaign aimed at systematically conquering the island of Kyushu. In the same year, the province of Osumi was annexed and a war began with the Ito clan for control of the province of Hyuga. In 1577, Ito Yoshisuke was defeated and fled north, where he sought help from Otomo Sorin, the Christian daimyo of Bungo Province.
The battle for Kozuki Castle was a consequence of Oda Nobunaga's expansion in the Chugoku region. Toyotomi Hideyoshi was appointed to lead the campaign, which aimed to weaken the influence of the Mori clan in these lands. Under his command were famous samurai: Kuroda Kanbei, Takenaka Shigeharu, and Hachisuka Koroku. Amago Katsuhisa, who cherished the hope of one day restoring the Amago clan's lost dominance in western Japan, also joined Oda's army.
Despite his reputation as a failure, created by 20th-century popular culture, especially cinema, Takeda Katsuyori (1546–1582) was in fact a brave warrior and talented commander. However, he always remained in the shadow of his great father, the legendary Takeda Shingen. After Shingen's death, Katsuyori sought to at least equal him, if not surpass him. Therefore, his actions were not always determined by pragmatic calculation. This character trait greatly influenced the subsequent events that led to the demise of the Takeda clan.
The Buddhist peasant movement Ikko-Ikki, which emerged in the 15th century, had grown considerably stronger and larger by the middle of the following century. It had effectively turned into a large feudal army used for territorial conquests. The Ikko-ikki fought fiercely not only against other branches of Buddhism, but also against the provincial daimyo and the central government.
Hojo Ujiasu (1515–1571), one of the greatest representatives of the Hojo clan, spent almost his entire life at war. His possessions were in a difficult position: they were surrounded on all sides by aggressive neighbors ready to attack at any moment. In implementing his plans to expand the Hojo clan's influence in the Kanto region, Ujiasu was forced to confront well-known and powerful opponents—the Uesugi and Takeda clans. In the east, his main rival was the Satomi clan.
