Caernarfon Roman Fort, which was known as Segontium, was constructed around AD 77 to control the Mendai straits. Originally a timber fortification, it was rebuilt in stone in the second century AD and remained occupied until at least AD 365. Around that time a replacement fort, Hen Waliau, was built 130 metres to the west to provide coastal defence against Irish pirates.



Caernarfon is located at the western end of the Menai Straits, a 500 metre wide stretch of water that separates mainland Wales from the Isle of Anglesey (Mona Insula), and was also near the mouth of the River Seiont. When the Roman army first invaded Wales, around the mid-first century AD, the territory was controlled by the Ordovices tribe. Like other Welsh tribes, such as the Deceangli in north east Wales and the Silures in the south, they had warlike tendencies and were influenced to a greater or lesser extent by a druidic cult that was centred on Anglesey. As early as AD 47 the Romans invaded North Wales but military priorities elsewhere forced a withdrawal. In AD 60 General Paulinus led a new conquest of North Wales which included the storming of Anglesey and extermination of its druidic cult. However, once again military problems elsewhere led to Roman forces having to withdraw; in this case the outbreak of the Boudica rebellion in Eastern England which resulted in the destruction of a portion of the Ninth Legion (Legio IX Hispana) and the Roman settlements of Colchester, St Albans and London. Finally, in AD 77 General Agricola launched another invasion of North Wales which resulted in the defeat of the Ordovices tribe. It was during this campaign when Caernarfon Roman Fort was built.


Caernarfon Roman Fort, known to the Romans as Segontium (meaning the home of the Segontii), was built to sever the link between Anglesey and Snowdonia; the rich agricultural lands of the former meant it served as the region's primary source of food whilst the mountainous terrain of Snowdonia made it an ideal redoubt for insurgency against the Romans. The fort's location, just 200 metres from the River Seiont, meant it could control the trade routes across the Mendai straits and along the river whilst the fort itself could be easily resupplied by vessels of the Classis Britannica, the British arm of the Roman navy. The fort was garrisoned by a 500 strong Roman Auxiliary mixed infantry/cavalry regiment which, circa-AD 200, was the First Cohort of Sunici (Cohors Primae Sunicorum). When originally raised in AD 77 the fort was constructed from earth and timber and was laid out in the normal 'playing card' shape associated with Roman installations of that period. The defences enclosed an area of approximately five acres and consisted of a clay rampart topped with a timber palisade and fronted by a double V-shaped ditch. Inside the fort the Headquarters (Principia) was in the centre and was surrounded by barracks, workshops and storage facilities.


Segontium was destroyed by fire around AD 110. This may have been the result of enemy action but it is more likely to have been done by the Romans themselves. The army normally burnt forts on abandonment to deny their use to the enemy and to enable easy recovery of the iron nails that held the structure together (which were highly valued by local tribes). Around AD 130 the fort was rebuilt in stone and this new facility remained in use until at least AD 365 although it underwent a series of substantial modifications during that period. In particular in the late third century AD a large courtyard building, generally interpreted as an administration complex, and a bath house were built in the fort's south-eastern quadrant. This suggests the fort may have served as an administrative centre for North Wales although it is more likely this was done from Chester (Deva), base of the Twentieth Legion (Legio XX Valeria Victrix).


Around AD 365 Segontium was replaced by a new facility, now known by its modern name of Hen Waliau, located 130 metres of the west of the earlier facility. By this time a large portion of Segontium's garrison had been redeployed overseas; the Notitia Dignitatum, a written record of Roman military dispositions dated to around AD 395, records troops from Segontium in Slovakia. Accordingly Hen Waliau was much smaller than the earlier fort and enclosed just one acre. In accordance with late fourth century Roman military doctrine, it was built with substantial defences designed to resist a siege; its stone built walls were over 1.5 metres thick and augmented with bastions. Like similar forts at Cardiff and Holyhead, the primary purpose of the facility was to guard against the threat of Irish pirates who were routinely raiding the west coast throughout this period.


The fort probably remained occupied until the late fourth/early fifth century AD. Thereafter the site may have been adopted as a tribal centre and was mentioned in Historia Brittonum, a ninth century history. Segontium was also mentioned in the Mabinogion, a twelfth century document that recorded Welsh traditions and oral histories that were circulating at the time. One of these was the 'Dream of Macsen Wledig', a story about Magnus Maximus (Macsen Wledig), Roman emperor from AD 384 to AD 388 who dreamt of a journey overseas that ended in a Great Hall in a far away land where he saw a maiden of great beauty. He subsequently searched across the Empire to find this individual eventually arriving in Segontium. There he found Elen, the maiden in his dreams, who was the daughter of a Caernarfon chieftain and subsequently married her. The story, which may have some basis in fact, was exploited by Edward I during his conquest of Wales in the 1280s; Caernarfon Castle was built in imitation of the Roman Imperial capital of Constantinople (Istanbul).Furthermore, whilst constructing the castle, Edward's men 'found' the body of Magnus Maximus at Caernarfon and reburied him with a degree of pomp.





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What's There?

Segontium Roman Fort has been partially excavated and foundations of some of the buildings are on display. A museum attached to the site is now closed (seemingly permanently) but the grounds can still be accessed. Constantine Road runs through the site severing it into two parts.

Caernarfon Roman Fort Layout. The fort was laid out in the standard ‘playing card’ layout associated with Roman forts of the period. The name Caernarfon means 'the fort at the river mouth'.

Headquarters. The Headquarters (Principia) incorporated the administrative buildings of the regiment. This included offices, a courtyard for assemblies, a shrine for the unit’s standards and a strongroom.

Strongroom. A secure vault for holding the soldier’s pay.

North-West Gate. The fort’s North-West Gate.

Curtain Wall. When the fort was first raised, its defensive rampart was a clay bank topped with a timber palisade. The fort was rebuilt  circa-AD 130 with a new curtain wall constructed from stone abutting the original rampart.

Getting There

Segontium Roman Fort is reasonably well sign-posted. On-road parking is available. Grounds are accessed via Constantine Road; enter the museum grounds and enter the site.

Car Parking Option

Constantine Road, LL55 2LN

53.137071N 4.266181W

Segontium Roman Fort

LL55 2LN

53.137583N 4.265771W