Segontium is located in the town of Caernarfon in western Gwynedd. It was a Roman fort, which was inhabited right up until the end of the Roman occupation of Wales, and was garrisoned for most of that time by Roman auxiliaries from present-day Belgium and Germany. Segontium was the most important military base and administrative centre in this part of Britain.

The fort of Segontium was established by Agricola in AD 77 or 78 after he had conquered the Ordovices people of North Wales. The original timber defences were rebuilt in stone in the first half of the 2nd century. Archaeological research shows that, by the year 120, there had been a reduction in the numbers of men serving at the fort, and the size of the garrison continued to decrease through the 3rd and 4th centuries. Coins found at Segontium show the fort was still occupied at least until 394 AD. Walls, remains of the baths, and overall structure are still clearly visible on the ground.

The site was occupied by the native Welsh immediately after the departure of the Romans, and the Welsh kings and princes are known to have maintained a llys or palace at Caernarfon until the death of Llywelyn ap Gruffydd in 1282. Segontium is among the 28 cities of Britain listed in the History of the Britons, a chronicle traditionally ascribed to the Nennius. It is referenced in the prose of the Mabinogion, a collection of early medieval Welsh tales first collated in the 1350s. In one of its four tales, the ruler Macsen, identified with the Roman Emperor Magnus Maximus, dreams that he must find and marry the daughter of the current warlord occupying Segontium. Macsen is said to be the father of Constantine, who was the grandfather of King Arthur.

Throughout his reign, King Edward I appropriated for himself as much of the Arthurian legend as he could. His intent was to place himself in the lineage of Arthur as a means to legitimize his conquest of Wales. This is one of the reasons he chose to build Caernarfon Castle, his great seat in North Wales, on the waterfront below Segontium, with stones plundered from the site. He also, with great ceremony, supposedly reburied the bones of Macsen on the grounds of a local church, perhaps the adjacent Llanbeblig, though no evidence of that burial remains today.

Segontium, as the location of the former Llys of Prince Llywelyn, plays an important role in Crouchback, the first book in the Welsh Guard Mysteries.