Seisyllwg was a kingdom of medieval Wales, covering the areas now known as Ceredigion, part of Carmarthenshire, and the Gower peninsula. Origins 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… (More on Seisyllwg)