Samnyeonsanseong Samnyeon Sanseong Fortress ( literally " Three - Years ' Mountain Fortress " ) is the ruins of a Silla fortress and wall that was built beginning in the year 470 . The fortress was reconstructed in 486 . The ruins are located in present - day Boeun , South Korea . During the Three Kingdoms Period the town of Boeun - eup , located two kilometers from the fortress , was known as Samnyeon - gun or Samnyeonsan - gun and it is believed by some that the fortress gets its name from the town . However , the " Samguk Sagi " relates that the fortress was completed in three years ( Korean , " sam nyeon " ) , and that the fortress derived its name from this fact . This account also suggests that the nearby town derived its name from the fortress , rather than vice versa . The Annals of Joseon Dynasty show that the fortress was known as Ojeong Sanseong Fortress and was used during the Joseon Dynasty , probably because the fortress is located atop Mount Ojeongsan . The fortress has been registered by the South Korean government on the tentative list of sites awaiting approval as a UNESCO World Heritage site . Although parts of the old wall have survived to this day , it is undergoing a major reconstruction by the Korean government . It is also listed as Korean Historic Site No . 235 . The walls of Samnyeon Sanseong Fortress measured over 1 , 680 meters or 1 , 880 meters in length , depending on the reference source . They were built by alternating rows of slim and flat stones placed in horizontal and vertical layers . This method of construction resulted in a very strong wall structure . The walls of the fortress , built on vertical cliffs , combine to create an impenetrable wall which ranged from thirteen to twenty meters in height . The width of the walls ranged from five to eight meters . The walls of stone also incorporated ramparts of mounded earth . Other notable features of the ruins include four gates , seven Ongseong ( curved guard bastions ) , two sluice gates and five wells . The four gates , located approximately equidistantly measure at about 4 . 5 meters in length . The Ongseong , or guard posts , were placed in areas most vulnerable to attack and were 25 meters in circumference and 8 . 3 meters in height . A large pond was once inside the fortress and was used as an important water source although only the outline of the pond remains today . The fortress is valuable because it shows what building techniques were employed by the Silla during the late fifth century . Additionally , Samnyeon Sanseong Fortress exhibits the characteristics typical of a Korean mountain fortress . There are two general classifications of Korean - style mountain fortress . The Pokok style is a fortress which surrounds the valley while the Teimeui style is a fortress where the walls are built around the peaks of the mountain , sometimes described like a folding screen across the mountain . . This fortress follows the Teimeui tradition . The fortress occupied and protected a strategic location and is credited with helping Silla to achieve the unification of Korea . During the late seventh century , Silla used the fortress as a base to attack Baekje to the west while also preparing for advances against Goguryeo to the north . The fortress was pivotal in securing the Han River Valley , the central part of the Korean peninsula . . The reputation of the fortress is such that it is said that Wanggeon , the first king of Goryeo , failed in his attempt to take the fortress 918 . Excavations in 1983 of artifacts from the Three Kingdoms period up to the Joseon Dynasty give clues as to when the fortress was occupied and used . Thousands of tombs also surround the fortress . Another notable feature of the fortress is the presence of engraved calligraphy on a boulder at its entrance , writing that has been attributed to Kim Saeng .