What is "pengwern"

Pengwern was a Brythonic settlement of sub-Roman Britain situated in what is now the English county of Shropshire, adjoining the modern Welsh border. It is generally regarded as being the early seat of the kings of Powys before its establishment at Mathrafal, further west, but the theory that it may have been an early kingdom (or a sub-kingdom of Powys itself) has also been postulated. Its precise location is uncertain.