"(The greatest impact of castles) was to be experienced not in the realm of military affairs at all, but across the countryside, in the forests, the farms and the fields. Even the most prosperous of free peasants would soon learn to dread the sight of makeshift walls and towers being erected on a nearby hill. No more ominous silhouette could possibly have been imagined than that of a castle on a rock...the shadow cast by such a stronghold would invariably extend for miles. Never before had an entire generation of landlords come so suddenly into the possession of so lethal a coercive tool. Entire communities could now be dominated, cabined in, and controlled.
It is no coincidence, then, that those same decades which witnessed the sudden spread of castles over France should also have been distinguished by the systematic degradation of the peasant's right to roam. Woods and rivers, those primordial sources of sustenance, began to be ringed around with tolls, or placed off-limits altogether. Inexorably, the easier it became for a lord to enforce restrictions, and to privatize what had once been common land, the faster it occurred. The poor man out with his bow and arrow in the woods, tracking some game for his cooking pot, suddenly found himself branded a poacher, a criminal. No more hunting, shooting, or fishing for the peasantry. Those who wanted food would now have to work for it in the fields the whole year round. (Tom Holland, The Forge of Christendom: The End of Days and the Epic Rise of the West, p. 153.)
Another way to put this is that 9th century thugs built castles and proclaimed all the land around them to be theirs.
Prior to this, the peasants lived partly as hunter-gatherers, able to support themselves from the forests and streams. Food was free!
Suddenly, with the building of castles, some thug decided those were his forests and his streams, and, if peasants wanted food, they'd have to farm his fields. (This is one of the reasons that people in the years AD 900-1000 were generally taller and healthier than people who lived two or three hundred years later.)
The hooligan having established that the local sources of food really belonged to him, he fancied that he was an important person and deserving of some recognition--thus, the thug became a duke, an earl, a count.
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