For over a thousand years, the cultures of Greece and Rome dominated culture while the Celtic tribes kept a relative peace. But around 200, Germanic tribes invaded Europe and by 410 Rome itself had been sacked. The next few decades saw the Roman Empire quickly crumble, replaced with local kingships all over Europe who found fame and purpose in cattle raiding each other while their entertainers - bards and skops mainly, told tales about their accomplishments. This heroic age only occurred in Europe, and lasted until the establishment of strong monarchies at the end of the seventh century.
In Europe, the heroic age marked the end of an era that had been noteworthy for centuries of stable trade, government, roads, currency, and culture. The next few hundred years would be filled with famines, plagues, raiding, and conquests. At the center of all this was a simple chieftain with more warrior abilities and magnetism than his peers, Arthur. He was unable to rise above the limitations of his age, though, so he was never a great conqueror. He only controlled a few hundred square miles, just like his contemporaries Beowulf, Hrolf Kraki, and Sigurd.
Christianity had spread during the Roman period, but would become entrenched during the heroic age. While religion grew stronger, technology grew weaker. Knowledge of smithing, mathematics, and science would be lost for hundreds of years.
Rome was sacked in 410, weakening the prestige of Roman culture forever. In 451, Attila lost the Battle of Chalons-sur-Marne, and he died two years later. That allowed all the Germanic tribes to break away and initiated an era known as the heroic age in which famous people like Hrolf Kraki, Beowulf, Arthur, Sigurd, and Theodoric would thrive.
In 476 Rome fell. Its western holdings were quickly taken up by various tribes. Several had already occupied Britain, while the Visigoths took Spain and the Ostrogoths settled into Italy. The next few centuries would see the consolidation of Frankia, Spain, and Portugal as well as dozens of kingdoms in Italy, Germany, and Britain.
In an era where places of learning were looted and no government was stable, culture suffered. Many of the past century's advances and intellectual accomplishments were forgotten. They weren't lost though. Manuscripts became spectacles; the Lindesfarne Gospels are considered the most beautiful in the world. Metalworking and jewelry crafting continued though they were more primitive than they had been in the Roman period. And in monasteries throughout the continent, the works of Aristotle, Boethius, Augustine, and Cicero survived.
Homes were made of wattle and daub and armor was of leather. Medicine went from being an art and a science taught in Roman schools to a random group of potions and herbs learned from trial and error as well as superstition. During the Dark Ages, the pursuit of knowledge was considered witchcraft and scientists could be burned or drowned for their ties to the devil.