Before one can really understand anything about the Roman army, one first has to know at least vaguely how it is set up. So my first few posts will be concerned with organization. Today, I will focus on the legion structure, concerning how the overall legion was broken down and set up.
The Roman legion, as we know it today, did not really come into being until Octavian(Augustus Caesar) came into power and made his reforms before his death in 14 AD. Prior to this, there was no professional standing army. When there was a campaign or a defense, a legion was raised, and when the need had passed, the troops were disbanded. But by the time Augustus came to power and the republic was turned into an empire, the empire was simply too large to continue on with that organization.
When the violent civil wars at the end of the republic were concluded, there were over 60 legions that had been raised to fight. Augustus trimmed this number down to 28 legions, a much more manageable and affordable size. The legions kept their old names, so that instead of simply having the numbers 1-28(I-XXVIII), they had names such as III Augusta, III Cyrenaica, and the IV Macedonica, to name a few.
Unfortunately, there is not enough evidence to state categorically how many men were in a legion. Most modern estimates fall somewhere between 5000-6000 men. This includes all the parts of the legion except for the auxilia, which are a separate matter entirely.
(Note:Please keep in mind that the number and organization of the men in the legions changed greatly in the later Empire as well. For the purposes of not going insane, I am limiting most posts to the army of the first two centuries AD, with separate posts regarding later reforms.)
The breakdown of a legion, from the smallest to largest groups, is as follows:
Contubernium: A group of 8 men. These men share one living space, cook together, eat together, and fight together. While it had no significance on the tactical stage, it was the closest knit group in the legion, contributing to a strong espirit de corps in the legion.
Century: Ten contubernales formed a century, which, contrary to popular belief, had 80 men and not 100. Each century was still too small to be of any use on the battlefield, but centuries were useful in dividing up duties. The head officer of a century was the centurion, which will be explained in detail when I talk about the officers.
Cohort: Six centuries made up a cohort. The cohort was the functioning unit on the Roman battlefield-the generals gave their commands to be followed by the cohorts.
And finally, ten cohorts made up a legion, giving a total of 4800 legionaries.
Beginning at some point in the first century AD, the First Cohort, in some legions at least, was at almost double strength. Instead of the typical 6 centuries of 80 men, it was made of 5 centuries of 160 men, so that it’s strength would be 800 men. There is no evidence that states this is true for all legions at all times, however. It is possible that the First Cohort was only strengthened during war-time when extra troops were needed, but this, too, cannot be proven.
The Legionary Cavalry(Equites Legionis)
It is generally accepted that each legion, for the first two centuries AD, generally had about 120 legionaries serving as cavalry. These men were not part of their own century-like unit. Instead, they were allocated to different centuries, which seems to indicate that they were infantrymen upon enlisting, and were later transferred to the legionary cavalry without being removed from their old century. However, they still trained and marched together, and likely were under their own centurion, though that is still a matter of debate.
The Roman army had no separate specialist groups. Engineers, artillerymen, and other specialists were all individual members of their respective centuries. However, in light of their specials talents and expertise, they were given the status immunes. Immunes were exempt from duties such as fatigues(the dirty and exhausting jobs), guard duties, laboring, and any other similar task. While they had no extra pay for their specialist status, they were given the time each day to work on their specialist tasks.
Under the modern heading of engineers the Romans would have put a great many different jobs, all those that were involved in the process of building or maintaining forts or defenses, and responsible for laying roads and creating weapons and armour.
The very literate soldiers could be appointed to the administrative staffs of the officers. They would attend to financial and administrative duties, such as keeping records and attending to the pay of the legionaries.