What do we call the promise a lord gives to protect his vassal?

The story behind the answer

In the feudal contract, obligations were reciprocal: the vassal owed service and loyalty, while the lord owed defense and support. Homage and fealty are the ceremonies and oaths a vassal makes to a lord; allegiance is a broader loyalty to a sovereign. Only the lord’s side of the bargain is the guarantee to defend his man—militarily, legally, and often materially.

This duty went beyond showing up with knights. It meant refuge within the castle in wartime, representation in the lord’s court, and sometimes “maintenance” (land or income) to sustain the vassal. A lord who failed in this could be judged to have broken the feudal bond, freeing the vassal from service. These mutual promises helped stabilize a fragmented medieval world, turning castles into both fortresses and social safety nets when royal power was thin.