The Roman legionary fort Segontium was founded by Gnaeus Julius Agricola in 77 or 78, after conquering the Ordovic tribe in North Wales. It was the main Roman fort in this area, which could accommodate about a thousand auxiliary infantry. It was connected by a Roman road with a legionary fort at Deva Victrix (Chester). In the first half of the second century AD the original timber walls have been rebuilt into stone. Archaeological research shows that before year 120 there was a decrease in the number of troops in the fort. The inscription on the aqueduct from the times of Emperor Septimius Severus indicates that in the 3rd century, Segontium was already occupied by only 500 people from the 1st Sunicorum cohort. In the third and fourth century, the size of the garrison of the fort continued to decrease. At the time, its main role was to defend the shores of North Wales against Irish invaders and pirates. Finds in Segontium indicate that the fort was occupied until at least 394 AD.
Unlike the medieval castle of Caernarfon, which was built over the waters of the strait, Segontium was located more to the east, on a higher terrain, giving the opportunity to view the area.
The fort was erected on a rectangular plan with dimensions of 155 by 126 meters, with two main roads intersecting it: via praetoria and via principalis, dividing the stronghold into four quarters. Via pretoria took its name from the centrally located headquarter of the commander (praetorium) which it crossed. Via principalis was arranged perpendicular to it and ran right next to the building of commanders (principia) and headquarter. On both sides were the oblong buildings of the legionary barracks; two granaries and bathhouses were also erected in the fort.
The praetorium was a house consisting of rows of rooms opening onto the portico around a small inner courtyard or garden. The room with the plinth in the back of it could be a small temple. Principia consisted of a closed courtyard with portico, a large building, later divided to provide additional office space, and a row of five rooms at the back, of which the central was a chapel. The granaries were 27×6 meters in size. It were built according to an unusual structure without buttresses, only the floor beams took the whole mass of grain above. In the south-east corner excavations revealed a large building with a courtyard. It is possible that Procurator Metallorum had his headquarter in it, which was responsible for the mining of metal ores in the area. The building and the nearby bathhouse were demolished in the 4th century and replaced with a new bathhouse complex, but it seems that it has never been completed.
The whole fort was surrounded by timber and earth fortifications, and from the 2nd century AD a stone defensive wall, reinforced with corner towers. Similar twin towers flanked each of the four gates to the fort. The outer zone of defense were two separate ditches, 4.5 and 5 meters wide.
Until today, a large part of the foundations of the fort has been preserved, despite its being cut by the modern road. The remains of the fort and civil settlement along with the Roman temple of Mithra and the cemetery around the fort are visible.
Website castlesfortsbattles.co.uk, Caernarfon roman fort.
Website coflein.gov.uk, Segontium roman military settlement, Caernarfon.