Iestyn probably ruled Morgannwg for a little less than a decade (c. 1081–1090) and is believed to have built castles in the regions of Cardiff and Kenfig. The popular version of historical events is that Iestyn, following a dispute with his rival
Einion ap Collwyn, invited Robert Fitzhamon and his twelve knights into the region to settle the matter. Once invited in, they refused to leave. He was deposed c. 1090 by
Norman ruler
Robert Fitzhamon, lord of
Gloucester, who established a lordship based in
Cardiff and subsequently conquered the lowlands of Glamorgan (the
Vale of Glamorgan), which was parcelled out to his followers. The mountainous parts of Glamorgan were left in Welsh control.
Caradog ap Iestyn, the eldest son of Iestyn ap Gwrgant, was the only Welsh lord to retain lands in the Glamorgan lowlands after Fitzhamon had conquered them. He retained the land between the
River Neath and the
River Afan, and he and his descendants were known as the "
lords of Afan." His descendants are thought to include the Williams family of
Aberpergwm and the Powell Wilkins family of Carreg Cennen, Carmarthenshire. == Modern interpretation ==