The most revolutionary of all the Norman changes was the transplanting to England of that system of "land tenure based on military servitude" which had gradually grown up in Europe during the two centuries following the break-up of Charlemagne's empire - the so-called feudal system. Under it the ablest fighters received estates from the king and higher nobles in return for military service. In the chaotic days which followed the barbarian invasions in Europe, the rulers lacked money but had plenty of land. The only way for a noble to become wealthy and powerful was to secure a vast acreage; for, in the absence of modern industrial and financial capital, land was almost the only source of wealth.
(from A History of England and the British Empire)