Gauls
In 58 BC Julius Caesar launched the Gallic Wars and had conquered the whole of Gaul by 51 BC. He noted that the Gauls (Celtae) were one of the three primary peoples in the area, along with the Aquitanians and the Belgae. Caesar's motivation for the invasion seems to have been his need for gold to pay off his debts and for a successful military expedition to boost his political career. The people of Gaul could provide him with both. So much gold was looted from Gaul that after the war the price of gold fell by as much as 20%. While they were militarily just as brave as the Romans, the internal division between the Gallic tribes guaranteed an easy victory for Caesar, and Vercingetorix's attempt to unite the Gauls against Roman invasion came too late. After the annexation of Gaul a mixed Gallo-Roman culture began to emerge.
Vitruvius
At various locations described by Vitruvius, battles and sieges occurred. He is the only source for the siege of Larignum in 56 BC. Of the battlegrounds of the Gallic War there are references to: the siege and massacre of the 40,000 residents at Avaricum in 52 BC; Vercingetorix commented that "the Romans did not conquer by valor nor in the field, but by a kind of art and skill in assault, with which they [Gauls] themselves were unacquainted." The broken siege at Gergovia in 52 BC. The circumvallation and Battle of Alesia in 52 BC; the women and children of the encircled city were evicted to conserve food, where they starved to death between the opposing walls of the defenders and besiegers. And the siege of Uxellodunum in 51 BC. These are all sieges of large Gallic "oppida". Of the sites involved in Caesar's civil war, we find the Siege of Massilia in 49 BC, the Battle of Dyrrhachium of 48 BC (modern Albania), the Battle of Pharsalus in 48 BC (Hellas – Greece), the Battle of Zela of 47 BC (modern Turkey) and the Battle of Thapsus in 46 BC in Caesar's African campaign. A legion that fits the same sequence of locations is the Legio VI Ferrata, of which "ballista" would be an auxiliary unit.