The Mongchon Earthen Fortress was constructed using the natural hillock, which stood at heights of a maximum of around 45m above sea level. It is categorized as an early earthen fortress, much like the Wol Fortress in Gyeongju and the Dal Fortress of Daegu. The fortress extends 730m in a north-south direction, and 540m in an east-west direction. The layout of the fortress resembles a trapezoid. There is another hillock along the saddle inside the fortress, dividing the structure into four areas. There is a 270m-long outer fortress in the form of a straight line on the hill extending toward the northeast.
The earth was excavated or added to the natural hillock to form the fortress wall. Column holes of a wooden fence placed at intervals of 1.8m were found outside the northwestern and northeastern walls.On the outside of the fortress, there are moats along the bottom of the hillock, built using natural streams. The northeastern side of the fortress is surrounded by Seongnaecheon (stream), which formed a natural barricade.
A variety of relics were found inside the fortress. There is one ground building site, one hardened and heaped land site, nine pitted settlement sites, 31 pitted storage sites, two square settlements, seven columned settlements, and two pond sites. The relics that have been excavated are earthenware objects primarily formed around the mid or late 3rd century. In some pitted settlements, a small number of iron weapons, such as iron swords and iron arrowheads, were discovered, with suits of armor made of bones found in pitted storage facilities.
Considering the location and the relics recovered, it appears that this fortress played a very important role in terms of military and culture during the Hanseong Baekje Period. It is also assumed to be one of the fortresses that formed the capital of the kingdom in the early years of Baekje, along with the Pungnap Earthed Fortress.