The ruins of Hagi Castle are located at the seashore of Hagi city on the Sea of Japan at the delta of the Abu River. The castle was built on a low mountain, Mt. Shizuki, at the edge of the delta. Once an island, the mountain is now connected to the land by a sandbank.  The sea forms a natural moat at the back of the castle and a water moat connected to the sea protects the front. Although the keep and other wooden parts of the castle were demolished in 1874 shortly after the Meiji Restoration, the impressive stone walls and moat remain at the foot of the mountain. A steep path up Mt. Shizuki takes you to a secondary fortification where some original earthen walls remain. The top of the mountain was used as a quarry, and many large boulders still remain up there, marked with the indentations used for splitting rocks. 

The tenshu (keep) was a five-story structure, and many yagura (turrets) were constructed at intervals on the stone walls. On the mainland was a tertiary area, also protected by moats, for the higher-ranked retainer’s residences, and the castle town.

Hagi Castle was built in 1604 at the beginning of the Edo period as the main castle of the powerful Mōri clan. Its keep was a five-storey structure, and many watchtowers were built at intervals along the walls. Another area on the mainland, also protected by moats, was for residences of the higher-ranked retainers. The castle was the seat of the Chōshū Domain for over 250 years until 1863. The town of Hagi grew around the castle, and became a hotbed of radical opposition to the Shōgunate towards the end of the Edo period.

History
Originally a minor warlord from Aki Province, Mōri Motonari defeated local rivals to become the ruler of most of the Chugoku region of western Japan. However, the clan was subdued first by Oda Nobunaga, then by Toyotomi Hideyoshi. Under Toyotomi rule, the Mōri built Hiroshima Castle as their main stronghold. At the 1600 Battle of Sekigahara against Tokugawa Ieyasu, the Mōri backed the losing side, and were forced by the new Shōgunate to relocate to Hagi, then nothing more than a fishing village in a swamp. Construction of Hagi Castle began in 1604 and was completed in 1608. As Japan came under pressure to open to western trade in the latter 1800s, the Mōri clan moved their seat from Hagi Castle to Yamaguchi Castle for fear of attack by foreign naval forces after confrontations at Shimonoseki.