As you walk down towards Mt. Achasan from the summit of Yongmasan, you will find the ruins of a small fort of the Goguryeo era (BC37~AD668) built at a strategic point overlooking the area of Jungnangcheon Stream. On the way up, you will find numerous observation platforms from which one can see a great view of some of the landmarks of Seoul such as the Hangang Bridge, Mt. Namsan, 63 City building, and Gangnam COEX.