William the Conqueror's takeover of England in 1066 was very significant for the English language as well. The Old English spoken in England up until then was very similar to the early versions of Danish or German, and would have probably have developed in the same way that those languages did if no takeover had happened.
However, Edward the Confessor had been brought up in Normandy, France while Danish kings ruled England, and the French favorites Edward brought back were not popular with the Anglo-Saxon aristocracy (which is why Harold II became king immediately on Edward's death -- the nobles didn't want a Norman in charge).
Not only did Harold die in the Battle of Hastings, but William's armies fought and killed a large portion of the Anglo-Saxon aristocracy all over England. The lands and titles of the dead were given to Normans, so that many more French speakers had power in England than if William had been able to take over peacefully. (And they stayed French speakers, because they often commuted between their French lands and their English lands.)
It was three hundred years before the courts of England spoke English again, and in that period Old English absorbed so much Norman French that at first glance Middle English and Modern English look to be as close to the Romance family of languages as than the Germanic family from which they are structurally descended. One man's desire to be king influenced a thousand years of language.