Guard-penny, often translated as wardpenny, was a payment made in lieu of guard-duty. Although only recorded once in Domesday Book (CAM 1,18), it was one of the most common customary obligations in later times. The commuted guard-duties could be of various kinds: manorial, feudal or royal. Most commonly, they were for the maintenance of the public peace.
For more detail, see N. Nielson, Customary rents (1910).