The rise and fall of the ancient civilizations, especially Rome, created a power vacuum. No Rome, no peace, no stability, no centralized government. What rose from the fall was fractured cities, states and different people groups jockeying for their slice of the pie. This break up created what would become what we call Europe today. France, Germany, Spain, Italy, Croatia, Turkey, UK, Austria, Romania, Switzerland etc. are the direct descendant of the the break of Rome. People groups of similar ancestry and language started to secure and protect lands and cities for themselves and their own interest both politically and religiously. Some refer to this time as the Dark Ages. Its wasn't so much dark as it was Gray. It was a time of deep religious devotion, war, and vying for power. Gone was a time of centralized government pointing back to Rome, although not for a lack of effort by the Christian Church and the Pope.
During the middle ages a certain amount of classical knowledge was lost. Sophisticated forms of math, philosophy, poetry and engineering were no longer common knowledge. How to calculate the building of a dome, or the slope of an aqueduct over miles of terrain, the ancient languages were no longer understood, poetry, philosophy were lost. This loss of the knowledge and understanding of the classical world is what we call the Middle Ages, Medieval or Dark Ages.