Sønderborg is a town in southern Denmark at the narrowest point of the Als Strait, connected by bridge to the island of Als. It sits very close to the German border and has a history shaped by the conflicts between Denmark and Germany — the town changed hands several times between the two countries before finally becoming Danish again after the 1920 referendum.
The main sight is Sønderborg Castle, a former royal castle and later a German prison, which now houses a museum covering the history of the Schleswig region and the wars of 1848 and 1864. The waterfront area around the castle and the harbour has been developed into a pleasant promenade. Just outside town, the battlefield at Dybbøl is the single most important site in modern Danish history. The town is a good base for cycling around Als, which has a gentle landscape of farmland, beaches, and small harbours.