Ystrad Tywi
Coordinates: 51°52′05″N 4°15′18″W / 51.868°N 4.255°W Ystrad Tywi (English: Vale of [the river] Tywi) is an area of south-west Wales situated on the banks of the Tywi river as it approaches the sea to join the Bristol Channel at Carmarthen. Although Ystrad Tywi was never a kingdom itself, it was a valuable territory and was fought over by the various kings of Dyfed, Deheubarth, Seisyllwg, the Kingdom of Gwynedd, the Kingdom of Morgannwg and the Normans. The area now makes up most of the modern county of Carmarthenshire.
In the late 7th century king Seisyll of the Kingdom of Ceredigion conquered Dyfed and Ystrad Tywi. This enlarged kingdom of Ceredigion with Ystrad Tywi became known as Seisyllwg in his honour. In 894 Ystrad Tywi and Ceredigion were laid waste by King Anarawd of Gwynedd together with an English force from his ally King Alfred, in an attempt to regain the lands previously held by his father Rhodri Mawr.
In medieval times Ystrad Tywi was divided into three cantrefs: Cantref Mawr on the north bank; Cantref Bychan and Cantref Eginawc on the south bank of the river.
Ystrad Tywi was transformed into the county of Carmarthen when Edward I enacted the Statute of Rhuddlan in 1284 following the success of his war against Llywelyn. The commotes of Kidwelly, Iscennen and Carnwillion and part of Dyfed were added later to form the modern county.