The citadel was built in 1567 over a former royal castle (13C) and incorporates some of its features. It was slightly modified around 1670 by Vauban, who added the powder store, arsenal, and a half moon, on the city side, which protects the entrance. Pop into the two round towers (13C), Queen Berthe Tower (14C) and the chapel (18C). The casemates (19C) house a historical exhibition and the rampart walk affords lovely views of the Canche valley.