Why did the serfs work on the land for the vassalsHow Charlemagne changed Europe?

Serfs worked on the land for vassals primarily due to the feudal system, which established a hierarchical structure where serfs were bound to the land in exchange for protection and the right to cultivate certain plots for their own sustenance. They were often required to provide labor and a portion of their produce to the vassals in return for the lord's security and governance. Charlemagne significantly changed Europe by unifying large territories under a centralized authority, promoting education, and reviving trade and cultural exchange, which laid the groundwork for the development of modern European states. His reign marked a pivotal shift from localized feudalism to a more interconnected and organized political structure.