The crannog on Llangorse Lake near Brecon in mid Wales was discovered in 1867 and first excavated in 1869 by two local antiquaries, Edgar and Henry Dumbleton, who published their findings over the next four years. In 1988 dendrochronological dates from submerged palisade planks established its construction in the ninth century, and a combined off- and on-shore investigation of the site was started as a joint project between Cardiff University and Amgueddfa Cymru - National Museum Wales. The subsequent surveys and excavation (1989-1994, 2004) resulted in the recovery of a remarkable time capsule of life in the late ninth and tenth century, on the only crannog yet identified in Wales.This publication re-examines the early investigations, describes in detail the anatomy of the crannog mound and its construction, and the material culture found. The crannog's treasures include early medieval secular and religious metalwork, evidence for manufacture, the largest depository of early medieval carpentry in Wales and a remarkable richly embroidered silk and linen textile which is fully analysed and placed in context. The crannog's place in Welsh history is explored, as a royal llys ('court') within the kingdom of Brycheiniog. Historical record indicates the site was destroyed in 916 by Aethelflaed, the Mercian queen, in the course of the Viking wars of the early tenth century. The subsequent significance of the crannog in local traditions and its post-medieval occupation during a riotous dispute in the reign Elizabeth I are also discussed. Two logboats from the vicinity of the crannog are analysed, and a replica described. The cultural affinities of the crannog and its material culture is assessed, as are their relationship to origin myths for the kingdom, and to probable links with early medieval Ireland. The folk tales associated with the lake are explored, in a book that brings together archaeology, history, myths and legends, underwater and terrestrial archaeology.