Montreuil sur Mer is a walled town with a splendid Citadel. Parts of it date back to the 11th century but it was modified by Vauban into its present form. Beneath are underground magazines and casemates, used during by the British in World War I as a communications centre as Haig established the British HQ in the town.
Part of the Citadel is now a youth hostel and the site can be visited, but underground parts are sadly not accessible.