Pyongyang, currently located in North Korea, was removed from historical interest with the fall of Goguryeo in 668, and as a result, it had been almost a ruin for more than 200 years. However, Pyongyang reappeared at the forefront of history as Goryeo dynasty was founded on 918(the first year of King Taejo), and it had served as a advanced base to contain the northern forces with Goryeo"s identity of ‘succession of Goguryeo history’. What further boosted Pyongyang"s status is to claim to advocate ‘Seogyeong’.
In the early days of the Goryeo Dynasty, Gaegyeong, which was as an existing capital, and Pyongyang, which had been considered as a new capital relocation site after the unification of the three kingdoms, coexisted as Two-Capitals. Seogyeong was suitable for its role as a strategic base to protect the capital Gaeyeong from the northern forces. The reason for the change of Seogyeong’s status as a city behind of Gaegyeong was started when Donggyeong was established in Gyeongju. But the ‘Two-Capital system’ centered on Gaegyeong and Seogyeong continued because Donggyeong had gone off from a circumambulation after that. Since then, as Namgyeong had been constructed, a circumambulation of lean-to-one-side the one side Seogyeong had been extended to Namgyeong and re-established as the "Three-Capital system" of Gaegyeong, Seogyeong and Namgyeong.
It was the fortress system that showed Seogyeong"s status corresponding to that of Gaegyeong. The fortress of Seogyeong, which can be verified by materials, were ‘Jaeseong, Naseong, Wangseong, Hwangseong and Gungseong’. It is not easy to deduce the content because it is difficult to find materials that can tell the format and size of these fortresses. However, it is believed that the area where the fortress of Pyeongyang[Seogyeong) was located in the period of Goryeo and Joseon Dynasty was almost identical to the castle of Pyeongyang of Goguryeo. It was because it took advantage of the best example to make efficient use of the terrain and geography of Pyongyang[Seogyeong]. Therefore, it is highly likely that the Seogyeong in the Goryeo Dynasty consisted of the four-fortress system of ‘Jaeseong[Naeseong], Naseong[Ouiseong], Hwangseong[Joongseong], and Bukseong’.