In AD 77, the governor of Britannia, Cnaeus Julius Agricola, completed the Roman conquest of Wales by suppressing a rebellion of the Ordovices, and then capturing Mona - the Isle of Anglesey. The foundation of Segontium, overlooking the Menai Strait, can be linked to this event. With a series of other forts, linked by a strategic road network and supported by the legionary bases at Chester and Caerleon, it formed a critical part of the Roman method of control over the newly conquered Welsh tribes.
Archaeological excavations have shown that it accommodated a regiment of auxiliary infantry of up to 1,000 soldiers. Coins recovered from excavations show that it was garrisoned until about AD 394. Such a long occupation was unique in Wales, and was possibly due to the strategic position of the fort, controlling access to the fertile and mineral rich lands of Anglesey and by its later role in the defence of the Welsh coast against Irish raiders and pirates. Throughout the Roman period, Segontium was the military and administrative centre of north-west Wales.
From about AD 140, the fort was subject to prolonged piecemeal rebuilding of both its defences and internal buildings in stone. Further work undertaken in AD 200 and AD 350 reflect changes in the composition of the garrison.
Source: Davies, J.L. 2005. Segontium Roman Fort: CADW
M. Lloyd Davies, RCAHMW, 28 October 2008