Alfred's son and his three grandsons followed him on the throne in succession between the years 901 and 955. They were all brave, able, hard-working princes, the worthy offspring of such a progenitor. They carried out to the full the work that he had begun; while Alfred had checked the Danes and made them his vassals, his descendants completely subdued and incorporated them with the main body of the realm, so that they were no longer vassals, but direct subjects of the crown. And while Alfred had been over-king of England, his successors became over-kings of the whole isle of Britain, the suzerains of the Scots and the Welsh of Strathclyde, as well as of all the more southern peoples within the four seas. [Sidenote: =Edward the Elder, 901-925.--Incorporation of central England with Wessex.=] Alfred's eldest son and successor was Edward, generally called Edward the Elder to distinguish him from two later kings of his line. He was a wise and powerful king, whose life-work was the incorporation of central England, south of the Humber, with his realm of Wessex, by the complete conquest of the Danes of East Anglia and the Five Boroughs. When Alfred was dead, his Danish vassals tried to stir up trouble by raising up against Edward his cousin Aethelwulf, son of Aethelred. This pretender the new king drove out, and then, turning on the eastern Danes, slew their king Euric, the son of Guthrum-Aethelstan, and made them swear homage to him again. But a few years later the Danes broke out again into rebellion, and Edward then took in hand their complete subjection. His chief helper was the great ealdorman Aethelred of western or English Mercia, his brother-in-law. When this chief died, Edward found his widowed sister Aethelflaed, in whose hands he left the rule of the Mercian counties, no less zealous and able an assistant than her husband had been. It was with her co-operation that he started on his long series of campaigns against the Danes of central and eastern England. While Edward, starting forward from London, worked his way into Essex and East Anglia, Aethelflaed was at the same time urging on the Mercians against the Danes of the Five Boroughs. They moved forward systematically, erecting successive lines of "burghs," or moated and palisaded strongholds, opposite the centres of Danish resistance, and holding them with permanent garrisons.