In the county of Flanders, the emergence of medieval urban life is generally situated in the tenth century, the beginning of sustained growth in the late eleventh and the attainment of greatest demographic weight in the thirteenth century. As in other regions of north-western Europe, as a result of a commercial revolution and the extension and intensification of trade, the town developed as a distinct kind of social formation within feudal society. The merchant class played a fundamental role in shaping the social, institutional and topographical development of early medieval urban centres. In many parts of Europe, whether there had been continuity with Roman urbanisation or not, princely and seigneurial fortifications attracted merchants and artisans who usually settled on important river routes and formed a burgus or civitas. In his La production de l'espace, the French urban sociologist Henri Lefebvre called the medieval city a ‘space of accumulation’. Indeed, until c. 1300/1350, the Flemish towns and cities rapidly developed through a structural process of economic and demographic accumulation. Of course, the necessary condition for the growth of towns was rural development and accompanying population increase. Agricultural and demographic surpluses fed urban economic growth, quickening commerce and artisanal production, especially in regions where seigneurial power had diminished, and the movement of labour and spontaneous market development had become easier. This rise of the Flemish urban economy has become the historians' paradigm of strong and durable medieval economic growth. Since Henri Pirenne, many historians have focused on the rise of cities during the Middle Ages, often quoting Flemish examples.