It was not until a century late that one of these
kings, King Offa of Mercia (757-96) , claimed
"kingship of the English". He had good reason to do so. He was powerful enough to employ thousands of men to build a huge dyke, or earth wall,
the length of the Welsh border to keep out the
troublesome Celts. But although he was the most
powerful king of his time, he did not control all of
England.