Caernarfon Castle

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.

For around two centuries after the conquest of Wales, the arrangements established by Edward I for the governance of the country remained in place. During this time the castle was constantly garrisoned, and Caernarfon was effectively the capital of north Wales. There was a degree of discrimination, with the most important administrative jobs in Wales usually closed to Welsh people. Tension between the Welsh and their English conquerors spilled over at the start of the 15th century with the outbreak of the Glyndŵr Rising (1400–1415). During the revolt, Caernarfon was one of the targets of Owain Glyndŵr's army.