Monreuil-sur-Mer was an important mediaeval port but has not been ‘on the sea’ for many centuries. It was used in World War I as the Headquarters of General Haig but occupied by the Germans in World War II.
The latter built underground protected barracks beneath the western ramparts of the town. Two sets of barracks were planned, the northern of which was completed and the southern range left unfinished. Each set would have had two entrance passages, with cross passages at right angles providing accommodation and services. The remaining barracks covers an area of around 50m x 40m.