Boeun Samnyeonsanseong Fortress (보은 삼년산성)

Boeun Samnyeonsanseong is a 5th-century Silla mountain fortress whose stone walls still ring the entire ridge crest in remarkably complete form. The name — "three-year fortress" — comes from how long it took to build, and the precise dry-fit masonry has held for over fifteen hundred years. A 2.7-kilometer walking loop traces the wall around the summit, with sweeping views of Songnisan and the Boeun valley below. It takes about two hours at a relaxed pace and is significantly less crowded than better-known fortresses near Seoul. Pair with the nearby Beopjusa Temple in Songnisan National Park for a full day combining ancient defense and Buddhist heritage.