When the German invaders conquered the Western Roman Empire in the 5th century, they destroyed the professional Roman army and substituted their own armies, made up of warriors who served their chieftains for honor and bounty. The warriors fought on foot and lived off the countryside. As long as they fought one another, they needed no cavalry. But when the Muslims, the Vikings, and the Magyars invaded Europe in the 8th, 9th, and 10th centuries, the Germans found themselves unable to deal with these rapid-moving armies. First, Charles Martel in Gaul, then King Alfred in England, and finally Henry the Fowler of Germany provided horses for some of their soldiers to repel the raids into their lands. Feudalistic governments were certainly occurring in the 11th century (Backman, 45-53).
Feudalism was based on a legal agreement by which a lord granted land to a man in return for military service. Political and economic power was concentrated in the hands of lords and their vassals. Castles, each of which dominated the district in which it was situated, were the base for the exercise of that power (Bloch, 147). In the castle there were various offices for management of the governing of the territory as well as for the personal needs of the lord and his family. There were also the offices for the oversight of the personal lands, estates, and manors belonging to the king. The royal castle was the headquarters for all of the functions necessary to see that the territory and the lords own holdings were cared for. In many cases all were primarily agriculturally based operations, business of the king, just as his feudatory lords had lesser personal territories with castle and lands. Great and small, all needed proper oversight and management (Herlihy, 238-239).
Feudalism was basically a descending pyramid of service, with duties running both upwards and downwards. The more privileges you had the higher up on the pyramid you were.