The Achasanseong (fortress) is mentioned at first on the record that King Chaekgye of the Baekje Kingdom fixed up this fortress against the invading Goguryeo Kingdom. King Jangsu of Goguryeo invaded the royal castle of Baekje which was located south on the lower Hangang (river), and he captured Gaero, king of Baekje, brought him to this fortress and killed him. Silla, which has taken the lower Hangang area, used this fortress as an advance base in attacking Goguryeo. Goguryeo's General Ondal who had tried to defend Silla's attack died here by the arrow of Silla forces. There is a sad-love song that Princess Pyeonggang of Goguryeo who came to take the corpse of her dead husband, General Ondal. The song, originated from this Achasanseong, had been sung for thousands years thereafter.

The shape of Achasanseong surrounding upper part of hillside remains today, beginning from the point that is 200m high above sea level to the southeastern Hangang riverside.

When Baekje had its capital at Hangang basin, the country had to build fortress at Achasan to defend Gogureyo's southward advance, as this area was very high.

Especially, this fortress was an important military-strategic point together with Pungnaptoseong (earthen fortification) which is located across southern Hangang. Both were key places crucial to Baekje's destiny.

This fortress in the early stage seems to be the Adanseong, whichis well known in the Baekje era as written at Samguksagi (3 kingdoms history record).

This fortress was also called Achaseong, Janghanseong and Gwangjangseong as well as Adanseong. The fortress shows that Baekje, Silla and Goguryeo had offensive and defensive battles one another around Hangang for a long time.

A vestige of this fortress remained recognizable until 1960's. The whole watershed of Achasan, and the other watershed from the northern foot of that mountain at eastern Myeonmok-dong to the southwestern foot of Achasan, the location 4km long of a earth wall and a stone wall from somewhere to Mojin-dong field is known as Janghanseong that was built by Silla.

The fate of Achasanseong has changed many times for 1500 years. Even today, the fortress reveals a few earthen vessels and roofing tiles of the old times as if displaying its fate.

It has been designated and protected as historicsites # 234. It has been transformed into a public park where many people enjoy ascending as if they remember the toils of fortress defending forces.

The dig area of Achasan 4thBoruseong is located at northern peak of Achasan ridge. It is 200m round. There are building site/well site/forge facilities and others inside the oval (saddle-shaped) barrack. Boruseong is the first military ruins of Goguryeo kingdom discovered in South Korea, as its earthwares provedas dating back to Goguryeo era. It is regarded as valuable material in studying the historical southward expansion of Goguryeo. This fortress clearly testifies the fierce battles among Baekje, Goguryeo and Silla kingdoms to occupy the lower Hangang area.

Achasan was also called Mangusan or Yongmasan. Bongwhasan Mountain, which is located north of Sangbongdong terminal, was called Achasan beacon in the old days.
A total of 15 fortress ruins were discovered at Achasan. A vestige of castle wall was found between fortresses. The castle wall was used also as a pathway in the 3-kindgom era, and later used as a ranch wall in Chosun dynasty.

Coming down southward from Daeseong temple, an ancient tomb can be seen on a rocky hill. It is just a small hill, but the tomb is located at a place difficult to ascend. Considering that point, the buried person seems to have been a very important person. It is noteworthy that style of the tomb is similar to that of Jeokseokchong inside the complex of Janggunchong (general's grave) of Goguryeo, which is located at Jib-An (second capital of Goguryeo), China. Length of the grave is 250Cm from north to south, its width 80Cm, and height 100Cm. A stone was found close to the tomb which might have been used to block the entrance once dead body was put in the tomb.

There were a number of Buddhist temples at Achasan but they no longer exist. Remains of the stone pagodas were found here and there, hinting that this place had accommodated Buddhist temples. The 3-story stone pagodas found at eastern valley of Daeseong temple were newly restored and designated as a cultural asset. The style of raising roof eaves seemed to have followed Baekje style. However, the stone pagoda seems erected in Korea dynasty, in view that its body is made of one stone and that erection technique is practical.

General Ondal was born from a humble family but later became son-in-law of king of Goguryeo. He led the Goguryeo army in attacking Achasanseong. In the midst of battle, he was shot to death by arrows of Silla army. The legend says he became a rock, keeping his pledge that he would not return home country without winning victory. His wife, princess Pyeonggang, traveled far away up to here, to find corpse of her beloved husband. In deep sorrow, she was also transformed into a stone near her husband, the legend says.