This large fortress is 4km northeast of Cheongju, on the slopes of the mountain Uam-san. Originally built in the 1590s and renovated in the 18th century, it has walls up to 4m in height that stretch 4.2km around wooded hillsides, offering superb views of farms, mountains and the city. Its size makes it easy to imagine that it once housed three temples and several thousand soldiers and monks. Today, Korean families decamp to the fields outside the south gate for family picnics.
A hike around the top of the wall takes about 1½ hours along a route that is completely exposed and can be steep in parts. The easier direction to follow is counter-clockwise. From the bus stop, walk back along the road and look on the left for a paved path that ascends to the top of the wall. Along the walk, there are hardly any signs or resting places, and no food stalls, vending machines or toilets – so bring your own water and a sun hat. If you’re up for a challenge, follow the path beside the pond and up the steep hill on the right to do a clockwise circuit. There are restaurants and shops near the bus stop.
Hop on bus 717, 832, 913 from outside the intercity bus terminal to the Cheongju Stadium (청주체육관) bus stop, which is just after the five-storey golden pavilion. Transfer to bus 862 (₩1150, 30 minutes, hourly) which runs to the fortress. The last bus back to Cheongju from Sangdang San-seong leaves at 9.50pm.