Samnyeon Sanseong 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 , 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.