What do we call the system where land is worked in exchange for protection?

The story behind the answer

This was the medieval system of mutual obligations built on land-for-service: a ruler or lord granted a fief to a vassal in return for loyalty and military aid, while those living on the land owed labor, rents, and support in exchange for the lord’s justice and protection behind castle walls.

It flourished in Europe after the Carolingian collapse and amid Viking and Magyar raids, when local power, knights, and fortified manors mattered more than distant kings. Oaths of homage and fealty formalized ties, and the network of lords and vassals structured politics and warfare for centuries. Don’t confuse it with manorialism (the economic management of an estate), serfdom (the legal status binding peasants to land), or sharecropping (a later crop-splitting contract).