Final answer:
Medieval Europe implemented a hierarchy called feudalism, a system of land ownership and protection where lords granted land to vassals in exchange for military service and other duties. It began to weaken after a series of crises such as the Black Death and the rise of nation states.
Step-by-step explanation:
Medieval Europe implemented a form of hierarchy known as feudalism. This system was characterized by a series of reciprocal legal and military obligations among the warrior nobility, revolving around the three key groups of society: those who fought (nobles and knights), those who prayed (the clergy), and those who worked (the peasants or serfs). At the bottom of the hierarchy were the serfs, who worked the land and owed service to their lords. Lords and vassals entered into mutual agreements where vassals were granted pieces of land, or fiefs, in exchange for military service, among other obligations.
The weakening of this system began after events like the Great Famine, the Black Death, and the Hundred Years' War, which led to upheavals in the medieval social structures. Furthermore, the growing power of the merchant class in urban centers and the development of nation states by kings challenged the feudal order and paved the way for the modern state system.