Artillery Deployed Behind Heavy Infantry Line

Roman artillery did not always wait for walls. Vegetius, writing in the late 4th century, situates engines behind the heavy infantry line—“post aciem gravis armaturae”—so they can act in the field, not just at a gate [4]. The image is a battle plain in Gaul: cohorts in the center, engines to the rear, their arcs aligned over friendly heads.

This placement makes sense only in a world of standardized machines and trained crews. Vitruvius’ ratios ensure that a scorpion’s pull translates into a predictable bolt flight, minimizing the risk to friends. Carroballistae on carts can move into position as the line deploys; teams can lower frames, lock pins, and begin to shoot within minutes [1][9]. The sound behind the line is mechanical—windlasses turning, skeins creaking—before it becomes violent: the snap of release and the hiss of a bolt.

The doctrine implies a choreography. On a plain near Aquileia, a commander extends his right; engines shift behind the gap; contubernales haul crates of bolts from a wagon. The color of the moment is the scarlet of standards and the pale wood of the rampart planks laid flat to provide level beds. When a hostile charge builds, the engines speak first, reducing momentum; then the heavy line receives.

This is not novelty for novelty’s sake. Ammianus, writing about onagers “in modern times,” notes crews of eight winders plus a gunner and the cushions needed to tame recoil [3]. Such details make rear placement plausible—shock weapons able to fire over infantry and into dense formations or at field fortlets. Vegetius’ line acknowledges that reality and writes it into practice [4].

The positioning also connects siege and field. A tower built by artifices in camp becomes an observation post over the line; a testudo shed turns into a mobile screen for engines shifting under missile fire. Frontinus’ sensibility—solve the problem with the tool at hand—finds reflection in this deployment: if the problem is cavalry massing, send bolts; if it’s a stubborn barn turned redoubt, roll an onager up and crack it [7].

Trajan’s Column gives us the look of movement that makes such positioning feasible: carts, axles, mule teams rolling frames up behind formations. Without that mobility, “post aciem” would just be a phrase. With it, we can see the dust plume of wagons moving to their marks and hear the command to fire, followed by the rip of air as a heavy bolt passes overhead [9][11].