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Carlsburg (also Carolus-Stadt or Carolsburg) was a 17th-century fortified town in Swedish Bremen-Verden at the confluence of the Weser and Geeste rivers, at the site of modern Bremerhaven, Germany. Planned to compete with Bremen, the settlement did not prosper.
After the Thirty Years' War, the Swedish Empire gained the Lower Saxon Prince-Archbishopric of Bremen as a Swedish dominion. The status of the city of Bremen however was uncertain, and Sweden was unable to gain control of the town in two subsequent wars, 1654 and 1666. As Bremen itself was out of reach, a new town was built nearby, named Carlsburg after the Swedish king Charles XI.
The town was planned by Stockholm architect Nicodemus Tessin the Elder. Tessin planned the public buildings and streets. He designated 300 to 456 building grounds for houses. Constructions began on 11 June 1672.