Sangdang Fortress (Sangdangsanseong) sits on the slopes of Uam Mountain, 4 km northeast of central Cheongju. The stone walls stretch 4.2 km around the ridge, standing 3–4 metres high, with three intact gates and the ruins of several buildings within the enclosure. The trail along the top of the walls is the best walk in Cheongju — not difficult, but long enough to be satisfying, with views over the city and the surrounding hills.
Inside the fortress walls, a small traditional village has survived: a handful of old houses, a couple of restaurants serving Korean mountain food, and the quiet that comes from being 200 metres above the city grid. Spring brings cherry blossoms along the lower slopes; autumn colours the mountainside in red and gold. Free to enter; open year-round.