It is one of the largest surviving medieval fortified towns. Louis IX (reigned 1226-1270) conceived of the walled city (1,650 total meters of walls). He wanted a port to access the Mediterranean, and he needed a fortified town to protect crusaders, pilgrims and merchants, providing a safe haven from which to launch crusades, as well as a center for trade between the Levant and northern France. Soon after 1240 Louis IX began the construction of the Tour de Constance, the isolated tower on the northwestern corner of the site; it was finished in 1249, the year after he launched the Seventh Crusade. Construction of the walled town did not, however, begin until 1272 during the reign of Louis's son Philip III (reigned 1270-1285), and work continued into the early years of the 14th century. Aigues-Mortes was planned as a whole. In shape it is a slightly irregular rectangle, with streets laid out to a grid plan, derived from the Roman castrum. The demise of Aigues-Mortes occurred in 1481, when Provence became part of France and Marseille emerged as the major port of southern France.