The feudal systems were sustained because the 3 States of the Realm depended on each other. These were the social classes in which feudal societies were divided: clergy (1st state), nobels (2nd state) and peasants and burgeosie (3rd state). There was almost no social mobility between them. On top, states were ruled by a monarch with absolut powers.
The king divided his territories and gave them to landowners from the 1st and 2nd states. These also received the power to rule over the lands they were given. In turn, they had to pay to the king some taxes, both in terms of money and crops, to protect those lands in the name of the king and of course, to recognize him as their rightful ruler. In order to cultivate those lands, the landowners relied on the 3rd state. Peasants enjoyed the protection of the lords and worked for them. In exchange for protection, peasants paid most of the crops they grew to their lord as a tax for using their land. This is how the lord obtained wealth to pay his taxes and also for himself.
Peasants did a very hard work in deplorable conditions. But as they totally depended on their lord to get lands to cultivable for substainance, and on his protection, and the lord depended on them as working force to cultivate his lands, this interdependence generated a very strong system, really difficult to break.
