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Seisyllwg: Wikis


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Teyrnas Seisyllwg
Kingdom of Seisyllwg

Banner of the House of Ceredig Arms of Ceredig
Medieval kingdoms of Wales.
Capital Not specified
Language(s) Welsh
Government Monarchy
 - 655 - c. 690 Seisyll
Historical era Middle Ages
 - Established 680
 - Union with Dyfed forming Deheubarth 920

Seisyllwg was a kingdom of medieval Wales,[1] covering the areas now known as Ceredigion, part of Carmarthenshire, and the Gower Peninsula.



Seisyll (born c. 665) was a direct descendant of King Ceredig's eldest son, Usai. He was a late 7th century king of Ceredigion who invaded Dyfed and managed to conquer Ystrad Tywi. He enlarged the kingdoms of Ceredigion and Ystrad Tywi which then became known as Seisyllwg in his honour.

Expansion and Merger with Dyfed

By 800, the kingdoms of Builth and Gwerthrynion were united through intermarriage and conquest with Seisyllwg.[2]

The kingdom passed to Rhodri Mawr, the King of Gwynedd, in 872, and was from then onwards ruled by a branch of his family. In 920, Hywel Dda merged Seisyllwg with Dyfed to create the Kingdom of Deheubarth.[1] From this power base, he later went on to unite almost all of Wales.



  • Davies, John, "A History of Wales" (Originally: Hanes Cymru), Penguin Books, 2007, ISBN 978014028475


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