It is assumed that the fact that Naito Bizen Nyudo, also known as Nobusuke NAITO, who had been the hikan (low-level bureaucrat) of the Hosokawa clan became the Shugodai of Tanba Province facilitated the full-scale expansion of Yagi-jo Castle for the reason that Yagi-jo Castle displayed the power of the Naito clan which served as Shugodai.
細川氏の被官となっていた内藤備前入道こと内藤信承が丹波国の守護代となったことにより、八木城は守護代である内藤氏の威勢を示すものとして本格的に拡張されはじめたものと推定されている。