Gallic Wars
The Gallic Wars were a series of military campaigns waged by the Roman proconsul Julius Caesar against several Gallic tribes. Rome's war against the Gallic tribes lasted from 58 BC to 50 BC and culminated in the decisive Battle of Alesia in 52 BC, in which a complete Roman victory resulted in the expansion of the Roman Republic over the whole of Gaul (mainly present-day France and Belgium). While militarily just as strong as the Romans, the internal division between the Gallic tribes helped ease victory for Caesar, and Vercingetorix's attempt to unite the Gauls against Roman invasion came too late. The wars paved the way for Julius Caesar to become the sole ruler of the Roman Republic.
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.