According to early pedigrees he was a descendant of Ceredig ap Cunedda Wledig who is assumed to have been the first Brythonic ruler of Ceredigion to which he gave his name. At first king only of Ceredigion, Seisyll later added to his kingdom those parts of ancient Dyfed known as Cantref Mawr, Cantref Bychan, and Cantref Eginog, or collectively as Ystrad Tywi. Hence the name Seisyllwg was often applied to the enlarged dominion created by Seisyll, the Deheubarth of the central period of the Middle Ages possessed by the descendants of Cadell ap Rhodri the Great. Dynastic continuity was maintained in this area by Rhodri's marriage to Angharad, a sister of Gwgon ap Meurig, the last king of the old line of Seisyllwg (d. 871), who were both great-great-grandchildren of Seisyll.
Published date: 1959
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