Wales and the marcher earls
Wales is a land of hills and mountains. In the Middle Ages, its people lived mostly by rearing cattle and hunting. They spent much of their time fighting each other, or the English. But they were also fond of music and poetry. And they were proud of their ancient Welsh language.
There was never a king of Wales. Each district had its chief, who fought wars with his neighbours. Sometimes, one man made himself master of a large part of Wales. But when he died, his land was split up among his sons.
William I did not try to conquer the whole of Wales. He gave land on the Welsh border to his most trusted lords. We call them marcher earls or lords of the marches. (March means border.) They were allowed to build castles, raise armies, and fight as much as they liked with the Welsh.
The marcher earls conquered part of south Wales. They built castles (such as Cardiff and Pembroke) and brought in English peasants to farm the land. But strong Welsh chiefs, such as the Lord Rhys, could get the better of the marcher earls and trouble their king. Henry II made peace with Rhys in 1171, leaving him in charge of most of Wales.
Walter Robson: Medieval Britain; Oxford University Press, 1991/2000, page 35