The first actual record of a fortress built at Namhan-san is found in the History of the Three Kingdoms (三國史記 Samguk-sagi), written in the 12th century. The text states that Silla constructed a fortress called Jujang-seong (晝長城) in the 12th year of the reign of Silla King Munmu (672) and that the wall circumference measured 4,360 bo (步 paces). The Korean Peninsula had recently been unified under the Silla kingdom and tensions were mounting between Silla and Tang-China. Tang appeared ready to invade the Peninsula, prompting King Munmu to station some 40,000 troops in Pyeongyang in 672. Jujang Fortress was also built as part of the preparations to contain the Chinese in the northern part of the kingdom. A 2007 excavation project at Namhan Mountain Fortress uncovered an area where a massive building (over 50m long with walls about 2m thick) once stood. Some 20kg of roof tiles for the structure were also found. The building is presumed to have been for storing military supplies and to have been constructed in the reign of King Munmu.