The chronology of Robert's conquest of Glamorgan is uncertain, but it probably took place in the decades after he received the feudal barony of Gloucester. One explanation is the legend of the
Twelve Knights of Glamorgan, which dates from the 16th century, in which the Welsh Prince
Iestyn ap Gwrgan (Jestin), prince or Lord of Glamorgan, supposedly called in the assistance of Robert Fitzhamon. Fitzhamon and his Norman knights and defeated the prince of South Wales
Rhys ap Tewdwr in battle in 1090. As reward Robert took possession of Glamorgan, and "the French came into Dyfed and Ceredigion, which they have still retained, and fortified the castles, and seized upon all the land of the Britons." Iestyn did not profit long by his involvement with the Normans. He was soon defeated and his lands taken in 1091. Whether there is any truth in the legend or not Robert Fitzhamon seems to have seized control of the lowlands of Glamorgan and
Gwynllwg sometime from around 1089 to 1094. His key strongholds were
Cardiff Castle, which already may have been built, on the site of an old
Roman fort, new castles at
Newport, and at
Kenfig. His descendants would inherit these castles and lands. ==Founder of Tewkesbury Abbey (1092)==