The Basilica of Saint-Nazaire is an architectural gem combining Romanesque and Gothic styles. The Romanesque part dates from the 11th century, while the Gothic choir and transept were added in the 13th-14th centuries. It features exceptional 14th-century stained glass windows, among the finest in southern France. It was the cathedral of Carcassonne until 1801 when the seat moved to Saint-Michel in the lower town.