Roman Segontium was founded by
Agricola in AD 77 or 78 after he had conquered the
Ordovices in
North Wales. 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 medieval
Caernarfon Castle that was built alongside the Seiont estuary more than a thousand years later, Segontium was situated on higher ground to the east giving a good view of the
Menai Strait. The original
timber defences were rebuilt in stone in the first half of the 2nd century. In the same period, a large courtyard house (with its own small bathhouse) was built within the fort. The high-status building may have been the residence of an important official who was possibly in charge of regional mineral extraction. Archaeological research shows that, by the year 120, there had been a reduction in the military numbers at the fort. An inscription on an
aqueduct from the time of the Emperor
Septimius Severus indicates that, by the 3rd century, Segontium was garrisoned by 500 men from the
Cohors I Sunicorum, which would have originally been levied among the
Sunici of
Gallia Belgica. The size of the fort continued to reduce through the 3rd and 4th centuries. At this time Segontium's main role was the defence of the north Wales coast against
Irish raiders and pirates. Coins found at Segontium show the fort was still occupied until at least 394.
Medieval Segontium is generally considered to have been listed among the 28
cities of
Britain listed in the
History of the Britons traditionally ascribed to
Nennius, either as or .
Bishop Ussher cites another passage in Nennius: "Here, says Nennius,
Constantius the
Emperor (the father probably of
Constantine the Great) died; that is, near the town of Cair Segeint, or Custoient, in
Carnarvonshire". Nennius stated that the emperor's inscribed tomb was still present in his day. Constantius Chlorus actually died at
York; the Welsh monument might be for Constantine who was the son of
Saint Elen, the supposed patron of the
Sarn Helen. In the 11th century, the
Normans built a
motte nearby, whose settlement formed the nucleus of present-day
Caernarfon. Following the 13th-century
Edwardian conquest, the earlier work was replaced by
Caernarfon Castle.
Present day Although the
A4085 to
Beddgelert cuts through the site, most of the fort's foundations are preserved. Guidebooks can be bought from other Cadw sites, including Caernarfon Castle. The remains of a civilian settlement together with a
Roman temple of
Mithras, the
Caernarfon Mithraeum, and a cemetery have been also identified around the fort. == Mythology and fiction ==