The Three Castles was a former medieval lordship, comprising the fortifications of Grosmont, Skenfrith and White Castle in Monmouthshire, Wales. The castles were established by the Normans in the wake of their conquest of England in 1066, to protect the route from Wales to Hereford. Possibly commissioned by William fitz Osbern, the Earl of Hereford, they initially comprised earthwork fortifications with timber defences. In 1135, a major Welsh revolt took place and in response King Stephen brought the castles together to form the lordship, which continued to play a role in defending the region for several centuries.

Grosmont Castle is a ruined castle in the village of Grosmont, Monmouthshire, Wales. The fortification was established by the Normans in the wake of the invasion of England in 1066, to protect the route from Wales to Hereford. Possibly commissioned by William fitz Osbern, the Earl of Hereford, it was originally an earthwork design with timber defences. In 1135, a major Welsh revolt took place, and in response King Stephen brought together Grosmont Castle and its sister fortifications of Skenfrith and White Castle to form a lordship known as the "Three Castles", which continued to play a role in defending the region from Welsh attack for several centuries.