Third Largest Settlement You’ve Never Heard Of (In South Wales). In 1405 Grosmont was the scene of a major battle in the rebellion of Owain Glyndŵr. Rhys Gethin, Glyndwr’s ally, raised a force of around 8,000 men who descended on Grosmont burning the town to the ground. At the time Grosmont was the this largest settlement in South Wales but the battle saw the burning of maybe 100 homes. Grosmont never recovered. In retaliation, a force dispatched by Prince Henry who would become Henry V, and led by John Talbot, 1st Earl of Shrewsbury, Sir William Newport and Sir John Greynder from Hereford, intercepted the Welsh and defeated them, killing between 800 and 1000 men. Grosmont Castle, like its sisters White Castle and Skenfrith Castle, is a Norman castle built shortly after 1066 to protect the route from Wales to Hereford.