Caernarfon occupies a strategic point beside the River Seiont and Menai Strait within a Welsh landscape shaped by the Roman fort of Segontium, later settlement, and the court of the princes of Gwynedd. A Norman motte-and-bailey castle appeared after conquest in the late eleventh century, but Welsh rulers regained the area. Edward I began the present stone castle and a walled colonial borough in 1283 after defeating Llywelyn ap Gruffudd and invading Gwynedd. Construction asserted English military power over a conquered population rather than filling an empty frontier.

Master James of St George and a large workforce of masons, carpenters, quarrymen, smiths, laborers, and administrators created polygonal towers, banded masonry, curtain walls, gates, halls, kitchens, chapels, and royal apartments. The design may have evoked Roman imperial architecture and Constantinople, strengthening Edward's claims to authority. His son, the future Edward II, was born at Caernarfon in 1284 and later became Prince of Wales. Building continued intermittently for decades at enormous cost, but parts of the intended palace and gate complex were never fully completed.

The castle functioned as garrison, treasury, administrative center, and symbol of English rule. Welsh forces captured the town during the revolt of Madog ap Llywelyn in 1294 but not the completed stronghold; Owain Glyndŵr's forces attacked in the early fifteenth century. As the Tudor monarchy integrated Wales into the English state, Caernarfon's military importance declined. It later endured Royalist use and Parliamentary sieges during the Civil Wars. Domestic buildings decayed or disappeared, while robust towers and walls remained and attracted antiquarian, military, and civic interest.

Nineteenth- and twentieth-century conservation removed later accretions, repaired masonry, inserted stairs and roofs, and adapted portions for visitors and museums. The 1911 investiture of the future Edward VIII and the 1969 investiture of Charles as Prince of Wales renewed the castle's political symbolism and generated modern fittings and presentation. The main curtain walls and towers contain exceptional original medieval fabric, but vanished timber floors, roofs, furnishings, and unfinished sections mean the monument is not a complete royal residence preserved from 1283. Recent access projects add lifts, walkways, services, and carefully differentiated new work.

Cadw, the Welsh Government's historic environment service, cares for Caernarfon as a ticketed monument within the UNESCO World Heritage Site of Edward I's castles and town walls. Access depends on current opening and conservation conditions. Interpretation increasingly places architectural achievement beside conquest, displacement, borough privilege, labor, Welsh resistance, and the contested title Prince of Wales. The castle's power comes from genuine medieval stonework and landscape position, not from pretending modern ceremonies or visitor installations are ancient. Every repaired joint and new route must balance public use with a structure originally designed to dominate.