Samnyeonsanseong, Mountain fortress in Boeun County, South Korea

Samnyeonsanseong is a stone mountain fortress in Boeun County whose walls run about 1,880 meters along the ridge. The walls were built by alternating horizontal and vertical layers of flat stones, stand 13 to 20 meters high and 5 to 8 meters wide, and include four gates, seven curved guard bastions, two sluice gates and five wells.

Samnyeonsanseong was first built in 470 during the Silla period and rebuilt in 486. In the late 7th century Silla used it as a base against Baekje and Goguryeo, helping secure the Han River valley. The Samguk Sagi records that it took three years to complete, giving the fortress its name. It was designated Historic Site No. 235 in 1973.

The name comes from the Samguk Sagi, which records that the fortress took three years (sam nyeon) to build; some instead link it to the old town name Samnyeon-gun. At the entrance, calligraphy on a boulder is attributed to the Silla scholar Kim Saeng.

The walls run about 1,880 metres around the ridge, with four gate sites spaced roughly evenly and seven curved guard bastions at the most exposed points. The entrance boulder carries engraved calligraphy attributed to Kim Saeng. The pond inside survives only as an outline, and thousands of tombs lie on the slopes around the fortress.

Archaeological digs here have uncovered weapons, tools, and everyday objects from different time periods. These finds show how people actually lived and worked at this location across many centuries.