Battle of Pharsalus
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.