William the Conqueror innovated the feudal system, built castles and fortifications around the country, established a more advanced elite, and organised the country with the Domesday Book - all of this bringing England up to speed with other European kingdoms - but feudalism as an economic system had been around since the beginning of agriculture millennia before, although what we call the medieval feudal system had emerged roughly a century or two before the collapse of the Roman Empire. I've never before heard this idea that William the Conqueror "brought feudalism to England."

Life for the 7th century Saxon serf wasn't all too different to the late 11th century Norman English peasant, but the main difference once William the Conqueror invaded was that the feudal system gradually become more pronounced and organised, with economic practice becoming more manorial. When the Normans invaded in 1066, England had only been a united kingdom for little over a hundred years (before then it was divided into seven kingdoms), so essentially the country's economic structure became more organised under the Normans' administration. England began to adopt the Frankish tradition of knighthood. Reciprocal ties between monarchs, lords, knights, and serfs were more established, but the whole idea of medieval feudal system being a clear-cut division of power between these groups is a bit of an oversimplified myth. It's worth noting that the definition of 'feudalism' itself is a controversial topic among historians, with a lot of debate over its real economic and political definitions.