Namhan Mountain Fortress features multiple examples of Korean fortress wall construction techniques. Yeojidoseo (黎地圖書), a report on the geographical circumstances of Namhan Mountain Fortress, describes the site as a "fortress made in heaven." The interior is flat, while the outside is steep, like a crown atop a mountain.
The Annals of Taek Village (擇里志) also notes: "Namhan Mountain Fortress is low and shallow on the inside but high and steep on the outside. The Qing forces were unable to bring their weapons to bear and the fortress never fell during the Manchu Invasion of 1636. King Injo only left the compound for lack of food and the fall of Ganghwa Island."
The fortress wall, more than 12km in circumference, follows a rugged terrain over 500 meters above sea level. The geographical position makes the fortress difficult to attack, even
with many troops. The compound inside the walls is broad and flat, while water is plentiful: more than 80 wells and 45 lotus ponds. With a proper supply of food, tens of thousands of soldiers could be kept here.
Structurally, Namhan Mountain Fortress was equipped with the necessities for an installation of its kind in the Joseon Period. Besides the main wall are two outer walls (Bongamseong and Hanbongseong). Two observation towers are on the south side of the fortress. The main wall are protected by 5 extended outer works (甕城 ongseong), and more than 20 reinforced works were installed. Four main gates and 16 hidden gates (暗門 ammun, a gate hidden from outside view, used to secretly transport weapons, food, other items, and people without letting the enemy know) were built into the walls. On top of the main fortress wall is a low, shallow wall (=parapet) called yeojang (女墻). This feature makes the overall wall appear taller from outside and blocks the view inside. The yeojang consists of 1,940 merlons with apertures for the defenders to shoot through. Some of the aperture are pointing straight out for shooting at targets in the distance, while others slant downward to shoot at enemies close to the fortress wall. Inside the fortress wall were 125 guardhouses (軍鋪 gunpo), with caches of salt and charcoal buried in more than 90 spots between them. Normally, provisions and military supplies were maintained inside the fortress as well.
The main wall at Namhan Mountain Fortress used stones that were first employed in the building of Jujang Fortress during Unified Silla. The outer wall was constructed at different times than the main wall was. Thus, one can find wall construction methods used from the 7th century through the 17th century. Such a variety of styles at one location is an important asset for understanding how wall construction developed in Korea. These days, many TV dramas are filed with the walls as a backdrop, and they attract large numbers of tourists each year. The Annals of Namhan (南漢志) details the scale of Namhan Mountain Fortress: "The inner circumference is 6,290 paces (17.5 li), while the outer circumference is 7,295 paces (20 li plus 95 paces). These figures would convert to 7,854 meters and 9,108 meters, respectively assuming one Korean foot (ja) is 20.81cm long. There are 1,940 merlons on top of the fortress walls, five extended outer works, 16 hidden gates, 125 guardhouses, and 5 command posts (將臺).
Three different methods can be used to measure the circumference of the wall, depending on the reference point: (1) the base (基壇部) of the outside wall, (2) the inside base (基底部) of the yeojang, or (3) the center line of the yeojang top. The simplest and most common way is basing the measurement on the center line of the yeojang top, which would make the main wall (without counting the outer wall and ongseong) 7,545 meters long and the area within the wall 2,126,637 square meters. Add the auxiliary facilities and the total sale comes to 12.356 square kilometers.