Speculation had already arisen in early Joseon that the site where Namhan Mountain Fortress stands today was once a capital for King Onjo, the founder of the Baekje kingdom. Baekje established its first capital at Wirye-seong, north of the Han River, and subsequently moved to several other locations in the Han River Basin. The strategic advantages of the mountain and the frequent movement of the Baekje capital are the basis for the theory that a Baekje capital was here at one time.
The direct link between Namhan Mountain Fortress and King Onjo, however, started much later, in Joseon. A shrine to the Baekje founder was built here in 1638, right after the second Manchu Invasion. Sungnyeol-jeon (崇烈殿) is a hall that enshrines the spirit tablets of Baekje Founder / King Onjo and Yi Seo, who was in charge of the project begun in 1624 to rebuild fortress. Ceremonies are conducted here on the fifth day of the ninth lunar month. Thus the history of the Joseon-era fortress is intertwined with that of a much earlier kingdom.