The hostel in central Seoul has a lot to recommend it. The rooms are tidy and affordable enough for K-pop fanatics on a budget and families in need of lots of space on vacation. It’s perched at the base of Namsan, the scenic, leafy mountain peak in the heart of town. There’s even a rooftop with panoramic views of the city.

Just don’t try to go to the basement.

Namsan, with its winding trails and springtime cherry blossoms, has long been a top destination for tourists in Seoul. Not long ago, though, "going to Namsan” meant something different, something sinister.