The Roman subjugation of the tribes of Wales took over thirty years (AD 47- 77) in the face of stubborn resistance organised by leaders such as Caratacus. Thegovernor Agricola’s defeat of the Ordovician rebellion of AD 77 brought an end to large-scale campaigning, and the military history of Wales until the fourth century seems to have been one of tight control exercised from strategically placed legionary fortresses, at Chester and Caerleon, but with a diminishing infrastructure of lesser installations to maintain law and order.
Although we still lack the complete picture, the most intense period of garrisoning followed the final conquest, but within very few years some military units were moved to the north of Britain, and this process was accelerated in AD 110-25, partly in response to the new frontier arrangements decreed by the emperor, Hadrian. The advances made in Scotland under Antoninus Pius in the mid-second century necessitated further adjustments to the garrisons of Wales, including further commitment of legionaries; only a few forts in the middle of Wales and Segontium on the north coast were maintained, implying some need to control the adjacent populations.
Britain was divided into two provinces by Caracalla in the early third century, Wales being assigned to Britannia Superior. Although changes were made to the military presence, for example by returning legio II Augusta in strength to Caerleon, the low troop density of the second century continued, and after AD 230 it dropped further. The later part of the third century witnessed a new concern: defending the coast from seaborne marauders. The response was a Saxon Shore-style fort at Cardiff and refurbishment elsewhere. It is not clear when the legionary garrison at Caerleon was finally withdrawn, possibly by the end of the century, although it may have left a skeleton force there as late as the mid-fourth century.
The military dispositions in fourth-century Wales at Cardiff, Caernarfon, Holyhead and Caerhun seem to have been concerned with continuing incursions by Irish bands. Forden underwent a major change of plan in AD 364-78 in response to an unrecorded crisis inland. The remaining recognisably military posts seem to have been abandoned c.AD 393 when soldiers were needed to counteract a rebellion in Gaul. There is some evidence that troops may have been left to guard the towns of Carmarthen and Caerwent into the fifth century.