In 1066 William of Normandy crossed the English Channel from his French homeland to invade Britain. It was one of the most ambitious military undertakings of the Middle Ages, and one of the most important—culturally and politically. The strange islands now known as Great Britain were then little more than a mystery, at once more cultivated and more barbaric than anyone guessed, an odd and sometimes even eerie mix of Roman, Celtic, and Norse influences, a place ready-made for folklore and legends.
The warriors who met William the Conqueror were not simply a ragtag group of tawdry fighting men who painted themselves blue, as misrepresentations will sometimes have it. King Harald Hardrada was no mere chieftain, and his troops were mail-clad soldiers who, though unhorsed, were adept at close-quarters fighting with their long swords and body-protecting shields. On the Bayeux Tapestry (a 230-foot-long embroidered cloth depicting the Norman conquest of Britain) they're shown in combat formations only maintainable by trained warriors.
William won the Battle of Hastings by tactical advantage through his use of combined arms: his archers, cavalry and infantry combined to overwhelm Harald's footmen, who were less mobile, could only engage the enemy at close quarters, and were unable to defend against the charging horses or the men swinging down on them from above. Legend says Harald died when an arrow pierced his eye, but this could be mere symbolism to show the effectiveness of the Norman archers.
What is definitely not symbolic is that Harald was the last English king to die on English soil until the death of Richard III at Bosworth Field two centuries later, and that the Norman invasion was the last successful invasion of England by a foreign power. Ironically, it marked the end of Britain as a more or less insular community; while England's shores remained her own, the coming of the Normans opened her up for commerce and interaction with the outside world in a way she'd never been before, constantly defending herself from attacks as she had been wont to do.
After his conquest was complete, William crowned himself King William I of England. For a long time afterward, there was palpable tension between the Saxons and the Normans, especially among the nobility, though contrary to popular belief the Saxons were no more native to the Isles than their Norman overlords. Eventually the tension subsided as England found new enemies, and as the population became more homogeneous, the two races becoming one through marriage and a common language.
The High Middle Ages weren't confined to England, and they lasted a lot longer than the Norman Conquest, but the experiences of the British were similar to those of all Europeans throughout the period. Conquests cemented rather than upset local people groups throughout Europe, language evolved due to military and cultural impositions, and the major Continental powers (Italy, Germany, Spain, France, Russia) began to more closely resemble their future selves at the end of the Medieval and the beginning of the Modern period.
As Christianity began to establish more of a foothold, rulers realized the church's potential as an arm of the state. Sometimes the clergy and the secular leaders were at odds, but nonetheless the established Catholic Church became an immovable element of European society, and was able to effect a degree of control the State by itself would find difficult to accomplish. At its best, the state church was a source of real spiritual guidance for otherwise ignorant peasants and nobles; at its worst, it empowered warriors to go on conquering sprees in the Middle East.
These expeditions, known as the Crusades, were essentially attempts at claiming the resource-rich and relic-packed Holy Land from the hands of the Muslim invaders. Part of this was simply a desire for wealth and honor, and part of it was the longstanding resentment of Europeans against the Muslim Moors who'd conquered much of the bottom half of Europe in the early Middle Ages. At any rate, the Crusaders justified an unholy undertaking with sanctified rhetoric, and killed, raped, and looted in the name of Christ.
In an ironic twist, it was actually the Muslims the Crusaders fought who most contributed to the glory of what would become Western Civilization. The Arabs were for centuries the custodians of higher mathematics, astronomy, philosophy, and literature, keeping much of what the Ancient Greeks had discovered and produced, and adding their own notations and commentary. The Crusading knights brought many of these back to Europe with them, and this cultural inundation laid the foundation for the Renaissance.
As feudalism began to evolve into an actual form of government and to merge with state policy and institutions, it became more and more necessary for people to acquire learning. At this point, they largely drew on the past, reminding themselves of what past thinkers and scholars had written, but it wouldn't be long before this backward-looking study would take shape as the Renaissance, and all of Europe and the world would be transformed by the second smallest continent which not too long before was awash in conquest, infighting, and religious superstition and crime.