Segontium was a Roman auxiliary fort on the hill above the Seiont estuary at modern Caernarfon, founded by Agricola around AD 77 during the conquest of north Wales and the subjugation of the Ordovices. It was a standard-sized auxiliary fort of roughly 2.3 hectares, garrisoned by a cohors of around 500 men, and remained occupied into the later 4th century — exceptionally long for a Welsh fort — with coin evidence suggesting activity to around AD 394, though structural reorganisation around 338 marks the conventional end of regular occupation.
Source: Pleiades — A Community-Built Gazetteer and Graph of Ancient Places. View the Pleiades record →
Segontium was the principal Roman military base in north-west Wales, controlling the Menai Strait, the approaches to Anglesey, and the coastal route, and serving as an administrative and supply hub for the region's wider network of forts and the copper and lead workings of Snowdonia and Anglesey. Its longevity makes it one of the few Welsh forts retained through the late Roman period, likely as part of the coastal defensive system against Irish raiding.
The fort was extensively excavated by R.E.M. Wheeler in 1921–23, revealing a sequence of timber and stone phases including barracks, principia, praetorium, and granaries, with later modifications including a Christian-period structure interpreted by some as a church. Finds include a notable Mithraeum located outside the fort (excavated 1959), inscriptions, coinage span
Segontium was a Roman auxiliary fort on the hill above the Seiont estuary at modern Caernarfon, founded by Agricola around AD 77 during the conquest of north Wales and the subjugation of the Ordovices. It is recorded in the Pleiades gazetteer of ancient places as a fort site from the Roman period in Britain.
Segontium is classified as a Roman fort — a military site in the Pleiades ancient world gazetteer. Roman Britain's archaeology encompasses thousands of sites ranging from legionary fortresses and marching camps to villas, temples and towns.
Several Roman sites lie within a short distance, including Segontium: Roman Bath House (0.1 km), Mithraeum at Segontium (0.2 km), Pen Llystyn (17.4 km). Aubrey Research maps over 2,200 Roman sites across Britain, drawn from the Pleiades ancient world gazetteer.
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