What was Charlemagne's greatest achievement?

Carolus Magnus ( the Latin name for Charlemagne) was king of the Frankish and Lombard people and was also the first of the Holy Roman Emperors. Since the fall of the western Roman Empire in 476 CE, power in Europe had been transferred to Byzantine rulers based in Constantinople. Charlemagne, through a series of political and military victories, became the first emperor to rule over western Europe since the 5th century. His dynasty, called the Carolingians, ruled for over two hundred years, while the Holy Roman Empire itself lasted over a thousand years. Charlemagne is sometimes called 'the father of modern Europe'.