used it as their headquarters during military campaigns. In 395 it became the seat of the Praetorian Prefecture of the Gauls, governing the western part of the Western Empire: Gaul proper plus Hispania (Spain) and Armorica (Brittany).
It became a favorite city of Emperor Constantine I, who built baths there, substantial remains of which are still standing. His son, Constantine II, was born in Arles. Usurper Constantine III declared himself emperor in the West (407–411) and made Arles his capital in 408.
Arles became renowned as a cultural and religious centre during the late Roman Empire. It was the birthplace of the sceptical philosopher Favorinus. It was also a key location for Roman Christianity and an important base for the Christianization of Gaul. The city's bishopric was held by a series of outstanding clerics, beginning with Saint Trophimus around 225 and continuing with Saint Honoré, then Saint Hilary in the first half of the 5th century. The political tension between the Catholic bishops of Arles and the Visigothic kings is epitomized in the career of the Frankish St Caesarius, bishop of Arles 503–542, who was suspected by the Arian Visigoth Alaric II of conspiring with the Burgundians to turn over the Arelate to Burgundy, and was exiled for a year to Bordeaux in Aquitaine, and again in 512 when Arles held out against Theodoric the Great, Caesarius was imprisoned and sent to Ravenna to explain his actions before the Ostrogothic king.
The friction between the Arian Christianity of the Visigoths and the Catholicism of the bishops sent out from Rome established deep roots for religious heterodoxy, even heresy, in Occitan culture. At Treves in 385, Priscillian achieved the distinction of becoming the first Christian burned alive for heresy(Manichaean in his case, see also Cathars, Camisards). Despite this tension and the city's decline in the face of barbarian invasions, Arles remained a great religious centre and host of church
