"This course travels through Hwarangdae Station – Mukdongcheon – Mangusan Mountain - Yongmasan Mountain - Achasan Mountain – Gwangnaru Station. The section along the ridge between Yongmasan and Gwangnaru Station commands a particularly nice view. In Mangu Cemetery, there are tombs of celebrities like Han Yong-un and Bang Jeong-hwan. The trail is also host to Achasan Fortress, dating back to the Goguryeo Dynasty (circa 37 BC – 668 AD)."
"Yongmasan Mountain (348m) is located between Mangusan Mountain and Achasan Mountain. According to legend, a child was born to a household. The child looked to be one of Herculean strength. The parents killed him, concerned that the child might grow to be a rebel. Thereupon, a dragon horse emerged from Yongma Peak and flew away. From that time on, the peak was called Yongma (literally meaning “dragon horse”)-san."
"Achasan Mountain (287m) is located between Gwangjin-gu, Seoul and Guri, Gyeonggi-do. Some people say that the name Achasan stems from Adanseong Castle of Goguryeo, while others associate the name with Hong Gye-gwan, a famous fortuneteller of the Joseon Period. A mountain range originating from Yongmasan forms Achasan, which extends southward to the Hangang River."
All the information above is cited from the official Seoul Trail website: http://english.seoul.go.kr/service/amusement/seoul-trail/03-seoul-trail-courses/#none