The first fortification on the site was a motte-and-bailey castle built in the late 11th century, which King Edward I of England began to replace with the current stone structure in 1283.

The castle and town established by Edward acted as the administrative centre of north Wales, and as a result the defences were built on a grand scale.

Despite its dilapidated condition, during the English Civil War Caernarfon Castle was held by Royalists and besieged three times by Parliamentarian forces.

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.

[6] This early castle was built on a peninsula, bounded by the River Seiont and the Menai Strait; it would have been a motte and bailey, defended by a timber palisade and earthworks.

[11] According to the Flores Historiarum, during the construction of the castle and planned town, the body of the Roman emperor Magnus Maximus was discovered, and Edward I ordered its reburial in a local church.

The earliest reference to building at Caernarfon dates from 24 June 1283, when a ditch had been dug separating the site of the castle from the town to the north.

According to a famous legend, the king had promised the Welsh that he would name "a prince born in Wales, who did not speak a word of English" and then produced his infant son to their surprise; but the story may well be apocryphal, as it can only be traced to the 16th century.

Even in peace time, when most castles would have a guard of only a few men, Caernarfon was defended by between twenty and forty people due to its importance.

[19] Edward I's campaign of castle-building in Wales cost £80,000 between 1277 and 1304, and £95,000 between 1277 and 1329;[20] by 1292 £12,000 had been spent on the construction of Caernarfon's castle—of which the southern façade was furthest along—and town walls.

The work was overseen by James of Saint George;[22] as a result, Walter of Hereford took over as master mason for the new phase of construction.

The accounts between November 1301 and September 1304 are missing, possibly because there was a hiatus in work while labour moved north to help out with England's war against Scotland.

[27] Such a sum was enormous and dwarfed the spending on castles such as Dover and Château Gaillard, which were amongst the most expensive and impressive fortifications of the later 12th and early 13th centuries.

[29] In Caernarfon's case the walls of the town and castle remained in good condition, while features which required maintenance—such as roofs—were in a state of decay and much timber was rotten.

Despite the disrepair of the domestic buildings, the castle's defences were in a good enough state that during the English Civil War in the mid-17th century it was garrisoned by Royalists.

The deputy-constable Llewellyn Turner oversaw the work, in many cases controversially restoring and rebuilding the castle, rather than simply conserving the existing stonework.

[32] Steps, battlements, and roofs were repaired, and the moat to the north of the castle was cleared of post-medieval buildings that were considered to spoil the view, despite the protest of locals.

[33] In 1911, Caernarfon was used for the investiture of the Prince of Wales, for the investiture of Edward, Prince of Wales (later Edward VIII), eldest son of the newly crowned King George V; the ceremony was held there at the insistence of the Chancellor of the Exchequer David Lloyd George, a Welshman raised in Caernarfonshire.

[40] A three-year restoration project, costing £5M and concluding in April 2023, has opened up previously-closed areas of the castle, and enabled wheelchair access to the battlements.

[41] The architect, Master James of Saint George,[42] was partly influenced by a desire to make the structure impressive as a symbol of the new English rule in Wales.

[44] It was divided into two enclosures, upper and lower "wards", in the east and west respectively, with the eastern containing royal accommodation, although this was never completed.

In the opinion of military historian Allen Brown, this combined to make Caernarfon Castle "one of the most formidable concentrations of fire-power to be found in the Middle Ages".

[47] Caernarfon's appearance differs from that of other Edwardian castles through the use of banded coloured stone in the walls and in its polygonal, rather than round, towers.

In his dream Maximus had seen a fort, "the fairest that man ever saw", within a city at the mouth of a river in a mountainous country and opposite an island.

Edward interpreted this to mean Segontium was the city of Maximus' dream and drew on the imperial link when building Caernarfon Castle.

[49] The purported discovery of Magnus's remains and the addition of carved imperial eagles to one of the towers may have been intended to reinforce this narrative.

[50] Recent work by historian Abigail Wheatley suggests that the design of Caernarfon was indeed an assertion of Edward's authority, but that it drew on imagery from Roman sites in Britain with the intent of creating an allusion to Arthurian legitimacy for the king.

[28] If the King's Gate had been completed, a visitor would have crossed two drawbridges, passed through five doors and under six portcullises, and negotiated a right-angle turn before emerging into the lower enclosure.

[53] In the opinion of architectural historian Arnold Taylor, "No building in Britain demonstrates more strikingly the immense strength of medieval fortifications than the great twin-towered gateway to Caernarfon Castle.

Though only the foundations survive, the Great Hall would have been an impressive building, featuring fine architecture, and used to host royal entertainment.