Tønsberg Castle, spread across the hill behind the town, was the largest fortress in Norway in the 13th century. In 1503 the Swedes destroyed what they could, but nonetheless, the modern (1888), 17m-high Slottsfjellstårnet tower provides a good viewpoint over the ruins. Parts of the 600m-long outer wall remain intact, while the extant medieval stone foundations include King Magnus Lagabøte's keep , the 1191 Church of St Michael , the hall of King Håkon Håkonsson and various guard towers . The park is always open.
