Roman Forts and Fortresses
Roman forts and fortresses (as opposed to camps) were the permanent or semi-permanent bases of Roman troops. These installations were a very important feature of the Roman period in Britain, as the British provinces were some of the most heavily militarised in the Roman Empire.
Permanent camps fall into two broad classes according to size and the troop type that use them. The army of the early empire was divided into two classes of troop: legionary and auxiliary. The legions were forces, each about 6,000 strong, composed of Roman citizens. The auxilia comprised units of infantry (cohortes), sometimes part-mounted (cohortes equitatae), and of cavalry (alae), with a nominal strength of 500 or 1,000, composed as a rule of men from the recently conquered tribes of the frontier provinces.
In the second century, numeri (units) come into existence, levies of troops differing in size, usually raised from conquered frontier territories and drafted for service as frontier guards in other areas from those in which they had been raised.
The legions were therefore relatively civilized, highly trained and expensive the auxiliaries relatively barbarous, rough and cheap. The Romans thus preferred to put auxiliaries in the forefront of a battle. Agricola, at the Mons Graupius, deliberately kept the legions in reserve in order to secure the vast renown of a victory in battle without Roman bloodshed; and such battles are actually illustrated upon Trajan's Column.
The auxiliary units were also expected to take the rough-and-tumble of frontier warfare. Accordingly, in fixing permanent quarters it early became usual to station the auxiliaries forward on the frontier, the legions in the rear behind a screen of auxiliary forts. Thus the small forts of the auxiliary cohorts and alae occupied a zone of territory along the edge of the Roman possessions, the larger fortresses of the legions were disposed behind this zone or at a strategical centre in it, at once protected against the first shock of invasion and in a position to strike at an enemy who should penetrate the frontier.