Sønderborg Castle sits on the waterfront at the south end of town, a thick-walled white quadrangle that has stood, in one form or another, since around 1170. Originally a Valdemarian fortress against the Wends, it became a royal residence, then a Prussian prison after 1864, and is now the Museum of Sønderjylland — the regional history museum, with strong sections on the Schleswig wars and the 1920 referendum that returned the area to Denmark.
The museum is unusually good for a town this size. The collection covers everything from medieval reliquaries to the Danish national-romantic painters who came here in the late 19th century to record a vanishing border culture. The castle chapel, in plain Lutheran white and gold, is one of Northern Europe's oldest princely chapels still in something like its original state.