[1] It traded mainly with Tang dynasty China and Japan, and served as a military hub to combat various pirate factions.

[2][3] Cheonghaejin was established originally as a military complex by General Jang Bogo in 828, the third year of King Heungdeok's reign.

Jang appealed to Heungdeok to establish a military complex in Cheonghaejin to protect Silla's merchant fleets and coastal residents from pirates.

Remains of various mercantile products and plates have been discovered in Cheongjaejin, which greatly contributed to understanding the life of Silla people.

The distance from Wando (island) to Jangdo is about 180m, which can be covered on foot twice a day when the ebb tide exposes the seabed.