The town of Sønderborg is home to Sønderborg Castle (Sønderborg Slot), the Royal Danish Army's Sergeant School (until 2013) and Sandbjerg Estate (Sandbjerg Gods). Sønderborg castle is in the centre of the town, and houses a museum focusing on the history and culture of the area. The museum is open all year. Sandbjerg Estate, which had belonged for many years to the Dukes of Sønderborg, and then to the Reventlow family, was donated to Aarhus University in 1954. In addition Sønderborg has a castle-like barracks built by the German military in 1906, placed centrally by Als Fjord, opposite Alsion (see picture below).
The old part of Sønderborg is on the island of Als, but some of its western suburbs have spread onto the mainland of Jutland into what had been the interior of the fort of Dybbøl.
The town of Sønderborg lies on both sides of Alssund; the narrow strait between these two sides is called Als Strait (Alssund). Two road bridges connect the city across the strait: the 682-meter Als Strait Bridge (Alssundbro), built in 1978-1981; and the 331-meter King Christian X's Bridge (Kong Christian Xs Bro), built in 1925-1930.