Goguryeo's Advance into the Downstream of the Yalu River and the Operation of the Defense System

The downstream of the Yalu River(鴨綠江) was a geographical location halfway between Liaodong(遼東) and the Korean Peninsula(韓半島). However, in the early days of Goguryeo(⾼句 麗)'s founding, the downstream area of the Yalu River and its center, West Anpyeong(⻄安平), were within the influence of the Han(漢). Therefore, it was necessary for Goguryeo, located in the middle and upper reaches of the Yalu River, to secure control over the downstream of the Yalu River in order to advance into the Yalu River and the Korean Peninsula. Accordingly, Goguryeo fought with the Han to claim sovereignty over this area. Meanwhile, when Goguryeo occupied West Anpyeong in 312, it was able to advance into this area in earnest, and the defense system for this area began to be established around the late 4th century. At this time, the main base in the downstream of the Yalu River was Daehaengseong(⼤⾏城). It is located in Dandong City(丹東) as a long flatland castle. It is presumed that the Daehangseong Fortress was a castle where a Rucho(婁肖) under the Bakjakseong (泊灼城) was dispatched to which Cheoryeogeunji(處閭近⽀) was dispatched. The defense system centered on Ogolseong(烏骨城), where Yoksal(褥薩) were dispatched from the downstream of the Yalu River, played a key role, and the defense system centered on Bakjakseong seems to have supplemented this. In addition, although a defense system centered on Dabeorakseong(多伐嶽城) existed in the downstream of the Yalu River, it is believed that it was not widely used compared to the defense system centered on Ogolseong and Bakjakseong. Like these Goguryeo built a defense system for the downstream of the Yalu River, centering on the land traffic route leading from Liaodong to the Yalu River. Meanwhile, in the case of the North Pyongan(平北) area connected to the Yaluchaek(鴨綠柵), a defense system was established centering on Yogiseong(辱夷城) located near the south of the Cheongcheon River(淸川江).