"Palladia Tolosa" was a major Roman city, with aqueducts, circus and theaters, thermae, a forum and an extensive sewage system. Protected by its walls and its distance from the Rhine, it escaped unscathed from the third-century invasions. With much of Gaul destroyed, Toulouse was the fourth-largest city in the western half of the Roman Empire after Rome, Treves and Arles. Around that time, Christianity entered the city; the Christian community expanded under Saint Saturnin (locally known as Saint Sernin), the first bishop of Toulouse, who was martyred in Toulouse about 250. In 313 the Edict of Milan established religious freedom in the empire, ending persecution of the Christians, and in 403 the Saint-Sernin basilica opened as a shrine for relics of Saint Saturnin.