In turn, the problem for royal authority was that many kings had “vassals” who had more land, wealth, and power than they did it was very possible, even easy, for powerful nobles to make war against their king if they chose to do so. The system was never as neat and tidy as it sounds on paper many vassals were lords of their own vassals, with the king simply being the highest lord. This system arose because of the absence of other, more effective forms of government and the constant threat of violence posed by raiders.
William claimed that Harold had pledged fealty to him, which justified his invasion (while Harold denied ever having done so). \): Depiction of a feudal pledge of fealty from Harold Godwinson, at the time a powerful Anglo-Saxon noble and later the king of England, to William of Normandy, who would go on to defeat Harold and replace him as king of England.