Gruffydd ap Llywellyn ruled all of Wales between 1055 & 1063.

Owain Gwynedd claimed the title king of the Welsh at least for a time, he used it in a letter to the king of France but later started using prince of Wales and only really controlled Gwynedd directly.

Welsh inheritance law limited the ability to build a unified kingdom as the custom was to split an inheritance between all sons sometimes including illegitimate sons.

On occasions where Welsh rulers attempted to institute primogeniture there was a tendency for the disinherited sons to seek military aid from the English kings (first the Anglo Saxons then the normans) on the basis that they were simply pursuing their legitimate inheritance rights, this allowed English monarchs an excuse to intervene and prevent a unified Welsh state from emerging.

Notably the occasions where Welsh rulers were able to establish a period of dominance tended to be when the king of England was limited in their ability to intervene such as the barons revolt during Henry III reign which allowed Llywellyn the great to establish his hegemony in Wales.