The historical centre of Sisteron deserves a thorough visit. The cathedral of Notre-Dame and St-Thyrse was built in the XII century and it is considered one of the greatest examples of the Provencal-Romanesque style with Lombard influences. The exterior is sober, in blocks of grey limestone, a round arch portal, and a wide rose window on the salient façade. The interior has a nave and two aisles, with an octagonal lantern and mighty pillars crowned by capitals with zoomorphic motives. There are valuable pictorial works dating back from the XVI to the XIX century. The town is dominated by the wide complex of its fortifications above it: the stronghold is composed by buildings from different epochs, standing on more ancient structures of roman and high medieval epoch, which are now vanished. The present castle dates back to a period from the XII to the XIX century. Vauban also worked on it and contributed to its planning after the invasion of the Durance valley by the duke of Savoy Vittorio Amedeo II. His ambitious project was only partly realized, in the powder magazine (powder keg) and on the northern side.