Auxiliary infantry soldiers were mostly organised in units about 500 strong, termed ‘cohorts’. Comprising six centuries, each commanded by a centurion, these were modelled on legionary cohorts.
Cavalry units, also about 500 strong, were termed ‘alae’ – literally ‘wings’, a reference to the traditional battle formation, where infantry were stationed in the centre and cavalry on the flanks. There were also double-strength cohorts and alae, along with units that had a mixture of infantry and cavalry.
An army record dated 13 July AD122 gives details of 13 alae and 37 cohorts that were stationed in Britain. At full strength, these units would have totalled around 30,000 soldiers, nearly a third of whom would have been horsemen. It is likely that a contingent of cavalry was stationed in the fort at London, but we do not know for sure.
Any free-born man could enlist in the auxiliaries. Many units originated as tribal militias in recently conquered parts of France, Spain or Bulgaria. Sometimes they were commanded by local warlords, who had been allowed to keep their armies in return for helping the Romans defeat their neighbours – with whom they were often already at war. Not long after the invasion in AD43, units of Britons were created. Most, if not all, were sent overseas to fight.
In the 2nd and 3rd centuries the auxiliaries were often supplemented by other units of irregular troops. It is known that there was an African regiment based on Hadrian's Wall. The Numerus Maurorum Aurelianorum is known to have arrived in Britain by the mid 3rd century. This 500 strong unit was based at a fort near Carlisle. Although most of the unit were probably Berbers , some may well have been negroes from the southern fringes of the African province of Mauretania. Other tombstones in the military north bear witness to other Africans, such as Victor, who was a Moor by race.
Except in mixed cohorts, the cavalry were organised in wings (alae). The ‘turma’, a troop of 32 horsemen led by a decurion, was the equivalent of the century. A standard ala comprised 16 turmae, giving it a total strength of just over 500. Cavalrymen at every level – from common trooper to commander – were better paid than their infantry counterparts, and had a much higher status.