This enormous complex, seat of the Dukes of Brittany, was constructed in the 9th or 10th century, enlarged in the 13th century, destroyed, then rebuilt into its present shape by François II in 1466. The Duchesse du Berry, royal courtesan, was imprisoned here, as was Gilles de Retz (aka “Bluebeard”), one of France’s most notorious mass murderers. The castle’s rich inventory has been presented as a museum since the 17th century. About 30 rooms are devoted to the history of the port, displaying evocative objects such as scale models of the city during different eras. The museum charges admission, but you can visit the courtyard and stroll along the ramparts for free.