The famous French engineer Vauban raised this citadel, a real master-piece that UNESCO listed as a World heritage site... It has a surface area of 11 hectares, with kilometres of surrounding walls... raised on Saint-Etienne mountain, the citadel was put up in 1668. But the construction stopped in the middle of the 17th century. Well, why?!
Because Franche-Comté was very troubled! It was the Ten Years War. France gave the region to Spain. But the French re-take it to the Spanish in 1668! Not for a long time: Franche-Comté immediately returned to Spain... But with the treaty of Nimègue, Franche-Comté région was finally annexed to France in 1678. The war was over and Vauban could finish his construction... which ended in 1711.