Under the Normans, the town came under the rule of the Marcher Lord (Lord of the March or border counties), and a fortification called Carreghofa Castle was built by the Earl of Shrewsbury around 1101. The place where the castle was probably built, at Tanat Camp, just to the west of Llanymynech Hill and overlooking the Tanat valley, implies it was defending the hill which was being mined for copper and lead, as well as silver. Being situated directly on the borderlands, the castle changed hands between the English and Welsh numerous times during the 12th and 13th centuries. In 1187 the castle was captured by Owain Fychan, prince of Powys, who was then murdered at the castle by his cousins Gwenwynwyn and Cadwallon.