Page "Roman legion" Paragraph 50

According to the late Roman writer Vegetius ' De Re Militari, each century had a ballista and each cohort had an onager, giving the legion a formidable siege train of 59 Ballistae and 10 Onagers each manned by 10 libritors ( artillerymen ) and mounted on wagons drawn by oxen or mules.

In addition to attacking cities and fortifications, these would be used to help defend Roman forts and fortified camps ( castra ) as well.

They would even be employed on occasion, especially in the later Empire, as field artillery during battles or in support of river crossings.