Founded by Celts almost 5,000 years ago, Carcassonne and its medieval hilltop citadel have been a UNESCO World Heritage Site since 1997. Today the old cité, France’s second-most-visited attraction, ranks only behind the Eiffel Tower in popularity—thanks in part to restorations by the famous 19th-century architect Eugène Viollet-le-Duc within the ramparts. Leave time for some shopping in the Bastide “new town” and a walk along the River Aude.