Castell Glas Castle Mound

The first fortifications at Caernarfon were built by the Romans. Their fort, which they named Segontium, is on the outskirts of the modern town. The fort sat near the bank of the River Seiont; the fort was probably built here due to the sheltered position and because it could be resupplied via the river Seiont. Caernarfon derives its name from the Roman fortifications. In Welsh, the place was called y gaer (lenition of caer) yn Arfon, meaning 'the stronghold in the land over against Môn'; Môn is the Welsh name for Anglesey.

Following the Norman Conquest of England, William the Conqueror turned his attention to Wales. According to the Domesday Survey of 1086, the Norman Robert of Rhuddlan was nominally in command of the whole of northern Wales. He was killed by the Welsh in 1088. His cousin Hugh d'Avranches, Earl of Chester, reasserted Norman control of north Wales by building three castles: one at an unknown location somewhere in Meirionnydd, one at Aberlleiniog on Anglesey, and another at Caernarfon.

War broke out again between England and Wales on 22 March 1282. The Welsh leader, Llywelyn ap Gruffudd, died later that year on 11 December. His brother Dafydd ap Gruffydd continued to fight against the English, but in 1283 Edward I was victorious. Edward marched through northern Wales, capturing castles such as that at Dolwyddelan, and establishing his own at Conwy. War finally drew to a close in May 1283 when Dolbadarn Castle, Dafydd ap Gruffudd's last castle, was captured. Shortly afterwards, Edward began building castles at Harlech and Caernarfon.