Roman legion (or individual legion units) when defending, when things went too far and the enemy was gaining the upper hand, they adopted defensive formations like orbis – a circle or agmen quadratum – a quadrilateral. Vegetius (4th century CE) also mentions aciesquadrata, i.e. an empty quadrilateral.
Some scientists believe that these formations were in fact very similar and were intended to defend their strength in the face of unfavorable conditions or the advantage of the enemy. Often the decision to form them fell on the lower echelons of command, in the face of a sudden attack.
According to the source sources, orbis used the legionaries of Quintus Titurius Sabinus and Lucius Auruncuileius Cotta in 54 BCE, when they were ambushed by the Eburons led by Ambiorix. However, the legionnaires were then supposed to adopt a quadrilateral rather than a circle formation. A similar formation was to be adopted by the troops of Julius Caesar at Ruspina in 46 BCE.
Regardless of the shape, however, the main purpose of the orbis, agmen quadratumor aciesquadrata was to ensure a successful retreat or enable the fight against superior enemy forces.
It is worth mentioning that the agmen quadratum was also a defensive marching formation when the supply wagons were in the middle of the marching group.
