|Founded||77 or 78 AD|
It probably takes its name from the nearby River Seiont, and may be related to the Segontiaci, a British tribe mentioned by Julius Caesar. The fort was founded by Agricola in 77 or 78 AD after he had conquered the Ordovices. It was the main Roman fort in the north of Roman Wales and was designed to hold about a thousand auxiliary infantry. It was connected by a Roman road to the Roman legionary base at Chester, Deva Victrix. Unlike the more recent Caernarfon Castle alongside the Seiont estuary, Segontium is located on higher ground giving a good view of the Menai Straits.
The original timber defences were rebuilt in stone in the first half of the 2nd century AD. An inscription on an aqueduct from the time of the Emperor Septimius Severus indicates that at that time it was garrisoned by Cohors I Sunicorum, which would have originally been levied among the Sunici of Gallia Belgica.
The site is now cut through by the A4085 road to Beddgelert, but the remains of most of the buildings are preserved. The visitor centre and small museum exhibiting finds made in and around the fort is now closed. Guidebooks can be bought from other Cadw sites, including Caernarfon Castle. Outside the fort, the remains of a civilian settlement have been found, together with a Roman temple of Mithras, the Caernarfon Mithraeum and a cemetery.
Segontium is implicit in the name of the surrounding town, because "caer" means fort. The name of the town of Caernarfon is the corrupted form of "Caer yn ar-Fon", which means "Fort in (the land) opposite Mon".
Segontium in mythology and fiction
In Breuddwyd Macsen Wledig ("The dream of Macsen Wledig"), one of the Four Independent Tales in the Mabinogion, Macsen (who can be identified with Magnus Maximus, who made a bid for Roman emperor in 383) dreams of a beautiful woman who turns out to be at "the fort at the mouth of the Seiont".
- Frances Lynch (1995) A guide to ancient and historic Wales: Gwynedd (HMSO)
- R.E. Mortimer Wheeler (1924) Segontium and the Roman occupation of Wales (Honourable Society of Cymmrodorion)
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