Koryo built a wall at the Yalu River in the eleventh century to help keep out what enemy?

In the eleventh century, Koryo built a wall at the Yalu River primarily to defend against invasions from the Khitan Liao dynasty. The Khitans were a powerful nomadic group from the north who posed a significant threat to the stability and security of the Koryo state. This fortification was part of Koryo's broader strategy to protect its territory and maintain its sovereignty against external aggressors.