Geoffrey of Monmouth completed his Historia Regum Britanniae around 1136, a pseudo-historical account of the kings of Britain that gave literary form to the Arthurian legend, including Arthur's court, his battles, his castle at Caerleon-upon-Usk, and his final passage to Avalon. The work was almost immediately translated into French and spread the Arthurian material across Europe, where it was developed by Chrétien de Troyes and others into the fully-fledged chivalric romance tradition. It is one of the most influential books ever written in Britain.

Geoffrey's Historia created the literary framework within which the castle — Arthur's halls at Caerleon, later transformed into Camelot by French romancers — became the defining setting of Western chivalric literature. This tradition directly shaped Malory, Spenser's Faerie Queene, Milton's early Arthurian plans, Tennyson's Idylls, and the castle imagery of a thousand poems in between. Every medieval and modern poet who used Arthurian material did so within a framework Geoffrey first sketched from a Welsh hillside.