Seisyllwg was a petty kingdom of medieval Wales. It is unclear when
it emerged as a distinct unit, but according to later sources it
consisted of the former
Kingdom of Ceredigion
Kingdom of Ceredigion plus the region known as
Ystrad Tywi. Thus it covered the modern county of Ceredigion, part of
Carmarthenshire, and the Gower Peninsula. It is evidently named after
Seisyll, king of
Ceredigion in the 7th or early 8th century, but it is
unknown if he was directly responsible for its establishment. In the
Seisyllwg became the center of power for Hywel Dda, who
came to rule most of Wales. In 920 Hywel merged
Seisyllwg with the
Kingdom of Dyfed
Kingdom of Dyfed to form the new kingdom of Deheubarth.
2 Later history and merger with Dyfed
It is unclear when
Seisyllwg emerged as a distinct unit. It is assumed
to have been named for
Seisyll ap Clydog, King of
Ceredigion in the
7th or early 8th century, and as such he is traditionally regarded as
Seisyll appears in the
Harleian genealogies for the
Kings of Ceredigion, but no early sources attribute the foundation
Seisyllwg to him, and the name
Ceredigion continues to be used into
the 9th century. The name
Seisyllwg appears in some later sources,
such as the Book of Llandaff, the Welsh Triads, and the Welsh laws,
the latter of which describes it as one of the three principal
subdivisions of South Wales, along with
Morgannwg and Reinwg (probably
Dyfed). However, the first clear description of the territory is
in the First Branch of the Mabinogi, where
Seisyllwg is said to
include the four cantrefs of
Ceredigion plus the three of Ystrad Tywi,
a description which accords with that in the laws.
Later history and merger with Dyfed
In 872, Gwgon, the last in the traditional line of kings of
Ceredigion, drowned, leaving no heir. Gwgon's sister, Angharad, was
Rhodri the Great
Rhodri the Great of Gwynedd, who became steward over
Gwgon's realm. While this gave Rhodri no standing to press a claim to
kingship himself, he was able to install his and Angharad's younger
son, Cadell, as the new King of Seisyllwg. Cadell ruled as a vassal
to his father, and later, to his elder brother Anarawd, who
established the Dinefwr family.
After Cadell's death in 911,
Seisyllwg was divided among his two sons,
Howel (later known as Hywel Dda, or Howel the Good), and Clydog.
Hywel probably already had control over the neighboring kingdom of
Dyfed by that time; there are no known kings of Dyfed following the
Llywarch ap Hyfaidd in 904, and Hywel is known to have been
married to Llywarch's daughter, Elen. He certainly had control over
it by the time Clydog died in 920, leaving the whole of Seissylwg to
Hywel. Hwyel quickly merged
Seisyllwg and Dyfed into the new kingdom
of Deheubarth, which covered most of southwest Wales. From this
power base, he later went on to unite almost all of Wales.
^ a b Davies, p. 85
^ a b c Lloyd, p. 257 and note.
^ Harleian genealogy 26.
^ Koch, p. 1602.
^ Jones, pp. 61–62.
^ Lloyd, p. 325.
^ a b c Lloyd, p. 333.
^ Koch, p. 945.
Davies, John (2007). A History of
Wales (Hanes Cymru). Penguin Books.
Jones, Basil (1851). "Vestiges of the Gael in Gwynedd". Archaeologia
Cambrensis, pp. 1–86. W. Pickering.
Koch, John T. (2006). Celtic Culture: A Historical Encyclopedia.
Lloyd, John Edward (1912). A History of
Wales from the Earliest Times
to the Edwardian Conquest. Longmans, Green, and Co. Retrieved J