This defensive structure, traditionally called an Ongseong, is an outwork designed to defend the curtain wall of Namhansanseong. There are five such facilities along the wall of Namhansanseong, each of which is further fortified by a battery set up at the end. The Second South Outwork is regarded as the most strategically important of all three outworks built along the south wall of the fortress, and has more gun platforms than the others, making it an effective defense structure against enemy attacks on both sides of the wall. The outwork is 318 meters in circumference and 134 meters in length, and covers 3,583m².
The Second and Third South Outworks were built with the traditional technique of filling the gap between the inner and outer parts of the wall with rubble, by which the inner part is sloped while the outer part is vertical. While the curtain wall of the main fortress was built by piling up irregular, or partly hewn, stones on the ground, the gun platforms were built by stacking larger stones on the bedrock layer.
Historians have discovered that the parapets built along the wall of the outwork underwent two or more renovations during the second half of the Joseon Period, although the majority of the structures built during this period have been destroyed. The latest excavations resulted in the discovery of roof tiles, square bricks and lime plaster used in the construction of the parapets.
The most recent excavation also revealed traces of ditches in four areas of the Third South Outwork and one in the Second South Outwork, as well as vestiges of stairs leading to the artillery batteries, i.e. two in the Third South Outwork and four in the Second South Outwork, and the base of an arched gate leading to the battery of the Second South Outwork. Other discoveries include lime-covered passages and stairways leading to the artillery batteries.