The construction of the rock fortress here on Obongsan Mountain was begun in 663, the third year of the reign of King Munmu (661-681) of the Silla Era (57 B.C.-A.D. 935). The fortress is believed to have been built to defend Gyeongju, the Silla Capital, from enemy attack from the west because historical records note that, during the reign of Queen Seondeok (780-785), troops of Baekje (18 B.C.-A.D. 660), a neighboring kingdom, advanced as far as Obongsan Mountain before they were driven back. It was all the more important because of its strategic location along the route to Daegu. Only a few structures, including the south gate of the fortress, remain. Today when most walls are heavily damaged, the sites of the grain storage, well, secret passage, and training field are still visible. This is famous as the place where Deuko, a member of the order of Hwarang (Flower of Youth, an elite youth military corps), wrote Mojukjirangga, an old Korean folksong about yearning for his comrade, Jukjirang.