By the summer of 52 BCE, Julius Caesar’s conquest of Gaul was threatened by the rebellion of a confederacy of Celtic tribes, led by Vercingetorix, king of the Averni. Caesar’s punitive expedition eventually cornered Vercingetorix in the hill-town of Alesia, in modern Burgundy. Alesia was well-fortified and garrisoned, and set between two rivers; Caesar therefore refrained from an all-out assault, building an encircling wall to prevent escape or resupply. Vercingetorix summoned support from his allies. Learning of this, Caesar built a second outer wall, concentrating his troops in redoubts between the two lines of defences. Spotting a weak point in the northern wall, the Celtic relief army attacked but was repulsed, as was a supporting sortie by the defenders. Finally Vercingetorix, supplies exhausted, ordered a mass sortie by his besieged forces, which the Romans defeated, decisively. Recognizing the hopelessness of his position, Vercingetorix surrendered, effectively ending the rebellion. He was taken to Rome in chains and displayed as part of Caesar’s triumphal parade before being executed; his men were massacred or sold as slaves.