Frederiksø is the third largest island in the Ertholmene archipelago located in the Baltic Sea. The island was named in honor of Frederick IV, King of Denmark and Norway. Frederiksø occupies an area of four hectares. The islands of Frederiksø and Christiansø that is located nearby share a yacht marina and a 30-meter drawbridge. Frederiksø is home to the 17th-century Lille Tårn, i.e. Little Tower, which now houses a museum. All buildings on the island are protected and car traffic is forbidden.