The Oberes Schloss (Upper Castle) crowns the Siegberg hill above the old town, a striking hilltop building in orange-red render that is visible from most of the city. The castle houses the Siegerlandmuseum, which covers two main subjects: the life and paintings of Peter Paul Rubens (born in Siegen in 1577) and the iron-mining and metalworking heritage of the Siegerland region. Both exhibits are well done, and together they give a good account of why this valley mattered beyond its modest current status.
The Rubens self-portrait is the headline object, but the museum's iron-working exhibits are arguably as interesting, particularly the collection of tools, models, and geological specimens from the Erzbergbau — the ore-mining operations that drove the regional economy from medieval times into the 20th century. The walk up from the old town takes 10–15 minutes through a pleasant park. The views from the castle terrace over the Siegen valley and the wooded Rothaargebirge hills are worth coming for even without entering the museum. Opening times and admission fees should be checked in advance, as they vary by season.