This multi-handed paper, written through seven decades, is a contribution to the history of medieval Morocco and to the historiography of archaeological research on this country. It includes a long unpublished text by Charles Allain (1920-2001) on the strategic medieval road linking Marrakech to Salé and which extended northwards to the Strait of Gibraltar and then al-Andalus. The author reconstructs the route of this vital axis of communication from the location and study of the hydraulic and civil engineering facilities that the Almoravid emirs and then the Almohad caliphs (XIth-XIIth centuries AD) built along, in regular stages. The data obtained reflect the interest of these sovereigns in land use planning and the control of communications, a thematic field that has so far been little explored by archaeologists. They are the result of an innovative approach for the time, the effectiveness of which is evident here. This text should have been the third chapter of a book which remained unpublished in 1956. It is preceded here by the preface initially planned by Henri Terrasse (1895-1971) for the whole book, as well as a few pages of presentation and context written for the occasion by the scientific editors (Patrice Cressier and Sophie Gilotte, archaeologists specialising in the medieval Maghreb).