Castle-guard was an arrangement under the feudal system, by which the duty of finding knights to guard royal castles was imposed on certain manors

Manorialism

Manorialism, an essential element of feudal society, was the organizing principle of rural economy that originated in the villa system of the Late Roman Empire, was widely practiced in medieval western and parts of central Europe, and was slowly replaced by the advent of a money-based market...

, knight's fees or baronies

English feudal barony

In England, a feudal barony or barony by tenure was a form of Feudal land tenure, namely per baroniam under which the land-holder owed the service of being one of the king's barons. It must be distinguished from a barony, also feudal, but which existed within a county palatine, such as the Barony...

. The greater barons

English feudal barony

In England, a feudal barony or barony by tenure was a form of Feudal land tenure, namely per baroniam under which the land-holder owed the service of being one of the king's barons. It must be distinguished from a barony, also feudal, but which existed within a county palatine, such as the Barony...

 provided for the guard of their castles by exacting a similar duty from their sub-enfeoffed

Subinfeudation

In English law, subinfeudation is the practice by which tenants, holding land under the king or other superior lord, carved out new and distinct tenures in their turn by sub-letting or alienating a part of their lands....

knights. The obligation was commuted very early for a fixed money payment, a form of scutage

Scutage

The form of taxation known as scutage, in the law of England under the feudal system, allowed a knight to "buy out" of the military service due to the Crown as a holder of a knight's fee held under the feudal land tenure of knight-service. Its name derived from shield...

known as "castle-guard rent", which lasted in to modern times.