At a glance
Free Cadw Roman fort at Caernarfon — occupied AD 77–383, one of the longest-garrisoned forts in Roman Britain. Exposed foundations and an on-site finds museum. 1 mile from Caernarfon Castle. LL55 2LN.
About Segontium Roman Fort
Segontium was a Roman auxiliary fort established around AD 77–78 on the edge of what is now Caernarfon — one of the northwesternmost points of the Roman Empire in Britain, commanding the approaches to Snowdonia and the crossing to Anglesey. The fort was garrisoned continuously for more than 300 years, until approximately AD 383, making it one of the longest-occupied Roman military sites in Wales.
The visible remains today are the exposed stone foundations of the fort's principal buildings: the principia (headquarters), the praetorium (commandant's house), granaries and barrack blocks — the layout of a standard Roman auxiliary fort laid out clearly in the ground. An on-site museum, run by National Museum Wales, displays three centuries of finds from the excavations: coins spanning every emperor from Vespasian to Gratian, pottery, inscriptions, metalwork and the objects of everyday garrison life.
Segontium's location is significant. It stood at the mouth of the River Seiont overlooking the Menai Strait and Anglesey — Mona to the Romans, the island the Roman general Paulinus attacked in AD 60 to suppress the Druids. The fort controlled both the route into the mountains and the sea crossing. Caernarfon's name derives directly from the fort: Caer yn Arfon, "the fort in Arfon". Entry is free; the fort grounds are accessible at all times.
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Frequently asked questions
Segontium (modern Caernarfon) was a Roman auxiliary fort established around AD 77–78 as part of the Roman conquest of northwest Wales under Julius Agricola. It was one of a network of forts controlling the approaches to Snowdonia and the Menai Strait. Segontium was unusual in its longevity: it was continuously garrisoned for over 300 years, until approximately AD 383 — one of the longest-occupied Roman military sites in Britain. The site today shows the exposed foundations of the fort's buildings, with an on-site museum displaying the archaeological finds.
The visible remains at Segontium include the exposed stone foundations of the fort's internal buildings — the principia (headquarters building), the commandant's house (praetorium), the granaries (horrea) and barrack blocks. The layout of a standard Roman auxiliary fort is clearly readable from the exposed foundations, making Segontium a good site for understanding Roman military architecture. The on-site museum (run by National Museum Wales) displays finds from excavations including coins, pottery, metalwork and inscriptions spanning three centuries of occupation.
Segontium was the administrative and military centre for Roman northwest Wales. Its position at the mouth of the Seiont river, overlooking the Menai Strait and Anglesey (Mona to the Romans — a centre of Druidic religion and resistance), made it strategically critical. The fort controlled both the route into Snowdonia and the crossing point to Anglesey. The garrison was typically an auxiliary cohort of approximately 500 men from various parts of the Roman Empire. A civil settlement (vicus) grew up around the fort to serve the garrison.
Segontium works well for children, particularly if combined with a visit to Caernarfon Castle. The scale of the fort foundations is legible and child-friendly guides to what the buildings were used for are available. The museum has coins, weapons and everyday objects that make the Roman period tangible. The site is free, open and not crowded — a useful contrast with the sometimes overwhelming scale of Caernarfon Castle. Allow 45 minutes to an hour, longer if using the museum.
In Welsh tradition, Segontium (Caer Seint) is associated with Magnus Maximus — the Roman general who led a rebellion in AD 383, withdrew troops from Britain to fight for the imperial throne, and in Welsh legend is remembered as Macsen Wledig. The Mabinogion tale "The Dream of Macsen Wledig" describes Maximus's dream of a beautiful woman at a fort at the end of a river, identified with Caernarfon. The fort was abandoned around the time of his rebellion — giving it a clear endpoint in historical and legendary terms.
Caernarfon Castle, built by Edward I from 1283, stands about 1 mile from Segontium — the Romans came first, followed 1,300 years later by the English. Edward I was aware of the Roman history of the area and reportedly incorporated Roman stonework from Segontium into the castle's construction. The town's name — Caernarfon, from Caer yn Arfon ("fort in Arfon") — refers to the original Roman fortification rather than the medieval castle. The two sites together cover 1,300 years of military history at the same strategic location.