Written by: Dave Galvin
Always bearing in mind that a single inscription or tombstone may not collectively attest to the presence of an entire cavalry unit, there are some 18 known to have served in Britain at one time or another. The greater number of 58 refers to mixed units of horses, infantry or specialist infantry only i.e. archers or scouts. The mixed units could be shuffled around to suit requirements whilst an ala, a wing of cavalry, remained predominantly just that.
The ala group consisted of approximately 500 men, a quingenaria. This was broken down into 16 turmae (squadrons), each of 30 men plus an officer (decurion), second in command (duplicarius, third in command (sesquiplicarius) and a standard carrier (signifer). A praefectus equituum commanded the group overall. The naming of a unit followed the patter either recording its place of recruitment i.e. Thrace (ala Thracum), or of recording its first recruiting commander i.e. Titus Pomponius Petra (ala Petriana). Larger cavalry forces existed, 1000 men strong (an ala milliaria yet these were scarce and allotted to certain provinces only owing to the cost of maintaining them and to the more pressing problem, that being a usurper using them against the legitimate armies of Rome. Britain had only the one serving Hadrian's Wall and its environs. This was the ala Petriana based at Stanwix (Uxellodumnum) the largest fort on the wall situated in the West.
There were smaller cavalry units (equites legionis), 120 men strong and these were allocated to the legions and acted as scouts and messengers. Another branch of the army was the Equites Singularis, a handpicked unit of horsemen chosen to protect either the Emperor or his Governors on their travels.
