Hwanhaejangseong

Hwanhaejangseong Fortress (Korean: 환해장성; Hanja: 環海長城), sometimes translated as the Hwanhaejangseong Fortress or the Hwanhae Great Wall, is a coastal fortification that once extended across much of the coastline of Jeju Island, today part of South Korea. Constructed in the 1270s during the Sambyeolcho Rebellion, the wall remained in use for centuries. However, in the late 20th-century it was heavily damaged, especially during the development of the island's coastal roads. In 1998, ten remaining stretches totaling 5,120 metres (16,800 ft) were designated a Monument of Jeju Province.

In 1270, during the Sambyeolcho Rebellion, the Goryeo government sent instructions for a wall to be built on Jeju to help prevent Sambyeolcho forces from taking the island. However, the Sambyeolcho attacked just two months after Goryeo forces arrived, taking the island. It is likely the Sambyeolcho continued to develop and extend the wall, before their eventual defeat in 1273. The wall remained in use, being seen as helpful in defence against Japanese pirates. In 1845, the walls were repaired and perhaps extended after a British ship spent a month near the island. The current walls are likely a result of this 1845 reconstruction.

The walls were built with local basalt, and the construction seems to have used traditional local techniques used to build other stone walls in Jeju. However, the precise form of the wall varied along its length. Attempts to repair and reconstruct the wall have faced challenges due to a lack of knowledge about initial construction techniques.