Carroballista
Carroballista was a sort of mobile field artillery that was cart-mounted. According to the Roman author Vegetius (Epitoma rei militaris II.25), each legion had 55 carroballistae (one percenturia), who were cheiroballistra arrow/bolt shooters. According to Vegetius, each carroballista was carried by mules and handled by a single contubernium (i.e., eight soldiers commanded by one decanus). The only surviving representations of a carroballista are in the bas-reliefs of Trajan's Column (Scene XL and Scenes LXV/LXVI) and Marcus Aurelius' Column.
The carroballista machine has the same structure as the cheiroballistra or manuballista. It appears that the technical advancements introduced by the Roman army's use of the cheiroballistra around the end of the first century AD enabled the employment of the cart. In fact, the light but strong metal spring frame and the broader area in the frontal half of the machine provided by the arched strut gave the ballista more maneuverability.
According to Trajan's column portrayal (Scene XL), the carroballista was driven by one guy mounted on the cart with the ballista and another man positioned behind the cart, most likely operating some form of the winch handle. The presence of mules in front of the cart implies that the carroballista could readily travel through the battlefield while firing bolts. Scholars do not all agree on this view.
The easy transportation of the ballista is represented in another section of Trajan's column (Scene LXVI), and now we have no arrows or bolts waiting to be released on the ballista, the ballista is empty, and no artilleryman is moving the machine. An artilleryman is hauling the cart near the wheel, implying that the entire mechanism must have been rather hefty.
It is certain that the cart was drawn by two mules or horses, and that it was presumably the size of normal Roman carts, i.e. c. 5 Roman feet (c. 147,85 cm) in breadth (as represented in Trajan's Column, Scenes XL and LXVI), although the cart's overall design is unknown. There are numerous speculations about the cart's structure, and different types of the same equipment appear to have been in use at the same time.
