The Orb

The Orb was a defensive measure used by a unit of the Roman Army to resist attack until reinforcements came.
If a unit became detached from the main Force and was surrounded, it formed itself into a circle and this was called the Orb.

The Contubernium (Tent group) was the smallest unit of the Legion, consisting of 8 men.
These 8 men were commanded by a Decanus, and it would have been his responsibility, if the unit became detached from the Centuria, and perhaps reduced in size, to form the Orb.
The men would unite with other detached Contubernia, until the Centuria could be reformed or rescued by a nearby Centuria.

Under the Cohort System, the Generals gave the junior officers their orders before the Battle.
The Cohors was big enough to successfully resist full frontal assault or cavalry charges on the flanks.
Unlike the previous Maniple System, the Cohors could revolve and face its flanks, or rotate and face an assault from the rear.
Each Cohors consisted 480 soldiers comprised of 6 Centuria of 80 men.
If the Centuries became separated, the Centurions could later regroup them when the opportunity arose.
The Centuria consisted of ten 8 man teams called a Contubernium or Tent group.
These 8 man teams were the smallest units in the Roman Legion and were each commanded by a Decanus.