The Collaborative International Dictionary
Castle-guard \Cas"tle-guard`\, n.
The guard or defense of a castle.
(O. Eng. Law) A tax or imposition an a dwelling within a certain distance of a castle, for the purpose of maintaining watch and ward in it; castle-ward.
A feudal tenure, obliging the tenant to perform service within the realm, without limitation of time.
Wiktionary
n. 1 The guard or defence of a castle. 2 (context UK legal obsolete English) A tax or imposition on a dwelling within a certain distance of a castle, for the purpose of maintaining watch and ward in it; castleward. 3 A feudal tenure, obliging the tenant to perform service within the realm, without limitation of time.
Wikipedia
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, knight's fees or baronies. The greater barons provided for the guard of their castles by exacting a similar duty from their sub-enfeoffed knights. The obligation was commuted very early for a fixed money payment, a form of scutage known as "castle-guard rent", which lasted into modern times. Castle-guard was a common form of feudal tenure, almost ubiquitous, on the Isle of Wight where all manors were held from the Lord of the Isle of Wight, seated at Carisbrook Castle.