Montreuil-sur-Mer (French pronunciation: [mɔ̃tʁœj syʁ mɛʁ] ⓘ; Picard: Montreu-su-Mér or Montreul-su-Mér; Dutch: Monsterole), Montreuil-on-the-Sea, is a subprefecture in the Pas-de-Calais Department in northern France.

Montreuil-sur-Mer is surrounded by notable brickwork ramparts, constructed after the destruction of the town by troops of Habsburg emperor Charles V in June 1537.

It was from a telephone and motor transit point of view in a very central situation to serve the needs of a Force which was based on Dunkirk, Calais, Boulogne, Dieppe, and Le Havre, and had its front stretching from the River Somme to beyond the Belgian frontier.

[7] Haig staff member Sir Frank Fox OBE wrote a critically acclaimed contemporary account of the headquarters in 1916, originally published under the pseudonym "GSO", called G.H.Q.

(Montreuil-sur-Mer) His work in the QMG's Directorate in the final offensive against the German Army resulted in his being awarded the OBE (Military) He was also Mentioned in Despatches.

However, the English-language version of the popular Schönberg-Boublil musical, and all screen adaptations thereof, restore the full name in the on-screen title cards, although it is never spoken in the libretto.