To resist invading tribes, Arles transforms her walls into a fortress.
During the early Middle Ages, the city was wracked by war and epidemics of black plague . When the Sarracens or Maurs invaded from the south, the inhabitants of Arles took refuge behind the Roman city walls.  The amphitheatre became a fortress. 
At the end of the 10th century peace and security returned and the city grew larger.  New sections appeared : the « vieux bourg » or old borough, presently known as the Roquette quarter, the « bourg neuf » or new borough and the « Cavalerie »quarter.
During the 12th century St. Trohime’s church and the living quarters for the canons were built around a cloister.  Arles knew a period of economic prosperity, brought by the pilgrims travelling to Santiago de Compostela along the Via Tolosana, also known as the « chemin d’Arles » or Arles route.  The Alyscamps cemetery became one of the largest Christian graveyards in the Western world.
The urban space was limited by a 13th-century wall surrounding the city and enclosing an area which did not change much right up until the modern period.  At the beginning of the 16th century, Arles, with its 10,000 inhabitants was the second largest city in Provence, after Marseille.