Moving the Capital to Ganghwa
To fight a long war against Mongol domination, Choe U moved the capital from Gaegyeong to Ganghwa Island, where the court remained for 39 years until 1270.

Jjw · CC BY-SA 4.0 · Source
The move exploited the Mongols’ weakness at sea. Palaces and offices modelled on Gaegyeong and new walls were built on Ganghwa, and tax grain from the southern rice lands came by sea. People on the mainland were ordered into mountain fortresses and onto islands.
The Mongols answered with a second invasion. The court on Ganghwa held out, but the mainland population suffered terribly from repeated invasions. The site of the Goryeo palace survives in Ganghwa town.