The Roman Army saw considerable changes during the period of Roman Britain. For most of the period the backbone of the army were the legions backed up by the auxiliaries. But major changes to the organization of the Roman Army occurred around about the end of the third century.
The legion had a nominal strength of over 5,000 men. They were divided into ten cohorts, nine of the cohorts had a strength of 480 men (split into six centuries each of 80 men), while the 1st cohort had 800 men. The legion also had a small unit of cavalry (120 strong).
The legionary was equipped with a short sword (gladius) used for stabbing, and two pila which were a heavy type of javelin. The soldier would be protected by armour and carry a large rectangular shield.
In command of the legion was the Legate; in the early empire he would have been a senator and a man of considerable seniority. The centuries were commanded by centurions who had provided the experience and much of the leadership; in battles they often suffered disproportionately heavy casualties. Some of the centurions worked their way up from the ranks, however others were appointed directly to the post.
Many of the legionaries were specialistsí engineers, masons clerks and medics. It was these highly skilled soldiers who built the Wall and played a large part in rebuilding it in the 160s.
There were usually three legions stationed in Britain. After the initial conquest of Britain, the legions were usually stationed in the legionary fortresses at York, Chester and Carleon (in South Wales). Obviously it must have taken some time for the legions to march up to the Wall, especially for the II legion based in Carleon or Gloucester.
The legions were responsible for building the Wall and frequently legionary detachments and sometimes entire legions were dispatched to the Wall. However they did not provide the main garrison for the Wall; this task was done by the auxiliaries.
The auxiliaries had originated as non Roman troops who fought alongside the Roman armies. They were particularly useful as skirmishers and cavalry (the Roman legions only having small cavalry units). The Emperor Augustus had formalized the situation and reorganized them into units (mostly 500 strong although some were double strength). There is no clear evidence for the strengths of the centuries in the auxiliary cohorts, but it is often assumed to have been 80 (the same as for the legionary centuries). Britain would have been home to over fifty auxiliary units, the highest concentration of any province.
Some of the auxiliary units retained their local forms of weapons and dress, and thus provided the Roman army with specialist weapons such as archers or slingers. However by the mid second century many of the auxiliary units would have had fairly standard equipment with the infantry heavily armoured and able to take their position in the main line of battle. On Trajan's column it is often the auxiliaries who are shown doing the fighting, while the legions are in reserve. This also seems to have happened at the battle of Mons Graupius where the auxiliaries seemed to have done nearly all the fighting with the Legions kept in reserve. This is in sharp contrast to events in the Boudiccan revolt where the Legions took the leading role.
The auxiliary units on the wall are surprisingly static, in many case the same unit is occupying a fort in the fourth century that it did in the late second century. The troops may have been recruited locally and have had little connection with the area where the unit originated.
The reward for honorable service in the auxiliaries was Roman citizenship, which would allow the soldiers sons to become eligible for service in the legions.
Auxiliary units came in three types - infantry, cavalry and mixed. The cavalry in the mixed units (cohors equitata) should not be thought of as mounted infantry they were proper cavalry units. All these types could come as normal(quingenaria) or double strength(milliara) units. There would have been over 50 auxiliary units in Britain at the time of Hadrian of which the most common would have been the mixed cavalry/infantry Cohors equitate. The top unit was the ala milliaria, there was only one in Britain, infact no province had more than one ala milliaria regiment. Command of this cavalry regiment would have been the pinnacle of an officers career and few would have made it this far. However the commander of this unit does not have exercised command over other regiments, all would have been under the nominal command of the commander at York.
While the strengths of the legion is well known, there are still some uncertainties over the strength of auxiliary units. The following are likely unit strengths. As you can see from the strengths of these units they would would have been very formidable units.
Cohors peditata quingenaria- 6 centuries (80 men) - total of 480 infantry.
Cohors equitata quingenaria - 6 centuries (80 men), 4 turmae of cavalry (32 cavalry men)- total of 480 soldiers and 128 cavalry men.
Ala quingenaria - 16 turmae of cavalry (32 cavalry men) - total of 512 soldiers.
Cohors peditata milliara - 10 centuries (80 men) - total of 800 infantry.
Cohors equitata milliara - 10 centuries (80 men) and 8 turmae of cavalry (32 cavalry men) - total of 800 soldiers and 256 cavalry men.
Ala milliara - 24 turmae of cavalry (32 cavalry men) - total of 768 soldiers.
In addition to the Cohors and Ala there was another kind of auxiliary unit - the Numeri. These were much more like the old fashioned auxiliary units, they fought in their traditional dress and were essentially irregulars. It's quite possible that they didn't have the organization of the other auxiliary units. Numeri were normally appear at the fringes of the empire and may well have served as a kind of frontier militia. They did not receive Roman citizenship on leaving the army.
The Army of the late Empire
By the fourth century the organization of the army had changed dramatically since the time of Hadrian, while the Legions had lost their pre-eminent status to the new mobile field armies.
In the fourth century the army was divided into comitatenses (a mobile field army) and limitanei (who were the troops based in the frontier areas). While this now seems to be a very radical change from the earlier organization, at the time it was probably seen as a natural progression. Over time the legionary bases had become fixed and increasingly the field armies were created from legionary detachments rather than entire legions.
Generally the legions (as well as all the other units under the dux Brianniarum) were part of the limitanei. At the same time the legions were reduced in size, for example the II Augustus is believed to have been reduced to about 1000 men in the 4th century. The comitatenses enjoyed higher status (and pay) than the limitanei.
The period also saw cavalry becoming increasingly important. The infantry were The infantry soldier was now equiped with a long sword (spatha), a spear and lighter throwing javelins rather than the gladius and pilum of his first century counterpart. By this time the soldier would carry an oval rather than rectangular shield.
Despite the changes in tactics, the auxiliary units garrisoning the Wall remained generally the same. So in many cases the same unit is stationed in a fort in the fourth century as was there at the beginning of the third century. However the strength of the units may have varied significantly.