Built literally around archaeological remains discovered during construction work in the late 1970s, this underground museum on Helgeandsholmen tells the story of medieval Stockholm through the actual ruins it preserves. You walk past a 55-metre section of the old city wall, remnants of a cemetery, and the foundations of buildings dating to the 1300s. The displays recreate medieval streetscapes with life-size figures and ambient sound, which could feel gimmicky but actually works well to bring the period alive. Admission is free, it rarely feels crowded, and it is an excellent complement to a walk through Gamla Stan just across the bridge.