Grosmont Castle was one of a trio of earth-and-timber strongholds built by the Normans in the aftermath of the Conquest to guard the communication route between Hereford and Wales, together with Skenfrith Castle (NPRN 93431) and White Castle (NPRN 94853). It lies within a wide moat (20m across), and a further second enclosure defined by scarps and ditches.
From the thirteenth century the site was re-built in the more durable local red sandstone, and timber defences were replaced by a stone curtain wall, protected by three circular towers and a gatehouse; a similar design to that of Cilgerran Castle (NPRN 95037). In the following century the buildings around the inner ward were remodelled to suit a noble household.
The castle was attacked and besieged by Gruffudd, son of Owain Glyndwr in 1405, before being relieved by a force from Hereford. By 1538 Grosmont was disused and abandoned. It came into state care in 1923.

Source: Knight, J.K. 2000. The Three Castles: Cadw Guide (Second Edition)

K Steele, RCAHMW, 3 November 2008.