The rocky hill that rises behind the town centre is Tønsberg's defining landmark. At the top stand the surviving foundations and walls of Tunsberghus — once one of Norway's most powerful medieval fortresses — and the 19th-century Slottsfjelltårnet tower, which you can climb for panoramic views over the town, the fjord, and the islands beyond.
The fortress itself was built in stages from the 12th century onwards and played a significant role in the power struggles of medieval Norway. It was besieged multiple times, most decisively in 1503, after which it was largely abandoned and gradually dismantled. What remains is atmospheric: low stone walls and rubble on a hilltop that still feels strategic.
The walk up from the town takes about 15 minutes and is steep in places. The tower is open in summer; admission is modest.