“Vale of (the river) Tywi”
Ystrad Towy, Ystrad Yw
Ystrad Tywi is a historical region in Wales located in the Tywi Valley, which is named after the River Tywi (Towy in English) that flows through it. The region encompasses parts of Carmarthenshire, Ceredigion, and Powys counties in southwestern Wales.
Here Arthur’s warriors fought a battle against the ferocious piglets of the boar Twrch Trwyth. On Arthur’s side, the warriors Peissawg the Tall, Llygadrudd Emys, and Gwrfoddu were killed, but the warriors managed to kill the piglet Llwydawg the Killer.
Ystrad Tywi | 0 to the 9th century AD
Roman Occupation | 1st – 5th centuries
During the Roman occupation of Britain, the area that encompasses the Tywi Valley was part of the Roman province of Britannia. The Romans established roads, forts, and settlements in the region. The town of Carmarthen (then known as Moridunum) was a key Roman administrative center and an important point on the Roman road network.
Decline of Roman Rule | 4th – 5th centuries
In the late fourth and early fifth centuries, as the Roman Empire declined, Roman control over Britain weakened. The Roman legions withdrew from Britain, leading to the fragmentation of the region and the emergence of indigenous Celtic communities.
Early Medieval Period | 5th – 9th centuries
With the Roman departure, the Tywi Valley and surrounding areas became part of the independent Celtic territories. The early medieval period was characterized by the presence of various Celtic tribes, including the Demetae, who inhabited the region. The local Celtic rulers established themselves as leaders in their respective territories and maintained some continuity of governance.
Formation of Welsh Kingdoms | 7th – 9th centuries
The Tywi Valley was within the broader region known as Deheubarth, one of the early Welsh kingdoms. Deheubarth encompassed southwestern Wales and included the Tywi Valley. In the ninth century, under King Rhodri Mawr (Rhodri the Great), Deheubarth expanded its influence and controlled a significant part of Wales, including the Tywi Valley.
Notes
The word ystrad is a general term for a low or flat valley through which a river flows, in Welsh. It’s possible the words strata, Latin; strath, Scottish; and the English street, are of the same origin. Sometimes the term ystrad was used to denote a paved road.
Source
Culhwch and Olwen | Late 11th century