Ystrad Tywi is an area of south-west Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

 situated on the banks of the Tywi river as it approaches the sea to join the Bristol Channel at Carmarthen

Carmarthen

Carmarthen is a community in, and the county town of, Carmarthenshire, Wales. It is sited on the River Towy north of its mouth at Carmarthen Bay. In 2001, the population was 14,648....

. Although Ystrad Tywi was never a kingdom itself, it was a valuable territory and was fought over by the various kings of Dyfed

Dyfed

Dyfed is a preserved county of Wales. It was created on 1 April 1974 under the terms of the Local Government Act 1972, and covered approximately the same geographic extent as the ancient Principality of Deheubarth, although excluding the Gower Peninsula and the area west of the River Tawe...

, Deheubarth, Seisyllwg, the Kingdom of Gwynedd

Kingdom of Gwynedd

Gwynedd was one petty kingdom of several Welsh successor states which emerged in 5th-century post-Roman Britain in the Early Middle Ages, and later evolved into a principality during the High Middle Ages. It was based on the former Brythonic tribal lands of the Ordovices, Gangani, and the...

, the Kingdom of Morgannwg and the Normans

Normans

The Normans were the people who gave their name to Normandy, a region in northern France. They were descended from Norse Viking conquerors of the territory and the native population of Frankish and Gallo-Roman stock...

. The area now makes up most of the modern county of Carmarthenshire

Carmarthenshire

Carmarthenshire is a unitary authority in the south west of Wales and one of thirteen historic counties. It is the 3rd largest in Wales. Its three largest towns are Llanelli, Carmarthen and Ammanford...

.

In the late 7th century king Seisyll

Seisyll

Seisyll ap Clydog was King of Ceredigion in Wales some time in the late 7th or early 8th century. He gave his name to the later kingdom of Seisyllwg, which consisted of Ceredigion plus the region known as Ystrad Tywi; as such he was possibly the king responsible for the expansion...

 of the Kingdom of Ceredigion

Kingdom of Ceredigion

The Kingdom of Ceredigion was one of several Welsh kingdoms that emerged in 5th-century post-Roman Britain. Its area corresponded roughly to that of the modern county of Ceredigion. The kingdom's hilly geography made it difficult for foreign invaders to conquer. Cardigan Bay bordered to the west...

 conquered Dyfed and Ystrad Tywi. This enlarged kingdom of Ceredigion with Ystrad Tywi became known as Seisyllwg in his honour. In 894

894

Year 894 was a common year starting on Tuesday of the Julian calendar.- Europe :* Northumbrians and East Angles swear allegiance to Alfred the Great, but promptly break their truce by attacking the south-west of England.* Mojmír II becomes King of Great Moravia after the death of his father...

 Ystrad Tywi and Ceredigion were laid waste by King Anarawd of Gwynedd together with an English force from his ally King Alfred

Alfred the Great

Alfred the Great was King of Wessex from 871 to 899.Alfred is noted for his defence of the Anglo-Saxon kingdoms of southern England against the Vikings, becoming the only English monarch still to be accorded the epithet "the Great". Alfred was the first King of the West Saxons to style himself...

, 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

Cantref Mawr

Cantref Mawr was a cantref in south-west Wales. It was of strategic importance in medieval Wales as the location of the main seat of the princes of Deheubarth at Dinefwr....

 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

Edward

Edward is an English given name. It is derived from Old English words ead and weard...

 I enacted the Statute of Rhuddlan

Statute of Rhuddlan

The Statute of Rhuddlan , also known as the Statutes of Wales or as the Statute of Wales provided the constitutional basis for the government of the Principality of North Wales from 1284 until 1536...

 in 1284 following the success of his war against Llywelyn

Llywelyn the Last

Llywelyn ap Gruffydd or Llywelyn Ein Llyw Olaf , sometimes rendered as Llywelyn II, was the last prince of an independent Wales before its conquest by Edward I of England....

. The commotes of Kidwelly, Iscennen and Carnwillion and part of Dyfed

Kingdom of Dyfed

The Kingdom of Dyfed is one of several Welsh petty kingdoms that emerged in 5th-century post-Roman Britain in south-west Wales, based on the former Irish tribal lands of the Déisi from c 350 until it was subsumed into Deheubarth in 920. In Latin, the country of the Déisi was Demetae, eventually to...

were added later to form the modern county.