​SONDERBURG, a seaport and seaside resort of Germany, in the Prussian province of Schleswig-Holstein, on the S.W. coast of the island of Alsen, of which it is the chief town, and 17 m. by steamboat N.E. from Flensburg. Pop. (1905), 7047. It is connected with the mainland by a pontoon bridge, and has a castle, now used as barracks, in the beautiful chapel of which many members of the Sonderburg-Augustenburg line lie buried; a Lutheran church and a town hall. There is an excellent harbour, and a considerable shipping trade is done. The town, which existed in the middle of the 13th century, was burnt down in 1864 during the assault by the Prussians upon the Düppler trenches.