Ystrad Tywi

'Vale of (the river) Tywi'
Ystrad Yw

A country in South Wales where Arthurís warriors fought a battle against the ferocious piglets of the boar Twrch Trwyth.

On Arthurís side, the warriors Peissawg the Tall, Llygadrudd Emys, and Gwrfoddu were killed, but the warriors managed to kill the piglet Llwydawg the Killer.

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.