Built in 1604 at the beginning of the Edo period as the main castle of the Mōri clan, it served as the seat of the Chōshū Domain for over 250 years until 1863.

The site was originally occupied by a minor fortification erected by the Yoshimi clan, who were based at Tsuwano Castle in Iwami Province.

The Yoshimi were retainers of the Ōuchi clan, who were shugo of the provinces on Nagato and Suō from the late Kamakura period.

In the mid-Muromachi period, the Ōuchi were overthrown by their own general, Sue Harukata, who was in turn defeated by the Mōri clan.

Under his grandson, Mōri Terumoto and with the support of Kikkawa Motoharu and Kobayakawa Takakage, the Mōri continued to expand, but eventually came into conflict with Oda Nobunaga, who general Hashiba Hideyoshi and gradually were forced back into the western half of the Chugoku region.

After the death of Hideyoshi, the Mōri supported his son, Toyotomi Hideyori, at the 1600 Battle of Sekigahara against Tokugawa Ieyasu.

Initially they petitioned to be allowed to construct their stronghold in either Yamaguchi or Hōfu, which were on the strategic Sanyōdō highway which connected Kyoto with Kyushu.

However, the shogunate rejected these proposals and ordered that they build their stronghold at Hagi, which was at the time only a small fishing village, distant from important roads and was a swampy area hemmed in by hills and rivers.