Montgomeryshire, Welsh Sir Drefaldwyn, historic county of north-central Wales, along the English border. Montgomeryshire is an area of wooded hills and valleys encircled by higher mountains, including Long Mountain in the east, Clifaesty Hill in the south, Plynlimon in the west, and the Berwyn mountains in the north. It extends to the Dovey estuary in the far west. Montgomeryshire lies entirely within the present county of Powys.
In prehistoric times important trade routes ran east-west along the course of the upper River Severn in eastern Montgomeryshire, and consequently that area has many hill forts dating from the Iron Age. The Roman occupation of the region was similarly centred in the east and was military in nature. Offa’s Dyke, the earthwork built in the 8th century by the Mercian kings to demarcate their kingdom from Wales to the west, runs north-south along the eastern border of Montgomeryshire. The area became the Welsh province of Powys in the 8th century ce. Powys was invaded by the Normans about 1090, and it fell to Roger de Montgomery in 1093. In the eastern part of Powys a string of Norman castles was built, including Powis Castle near Welshpool and one at the town of Montgomery. The area was under the rule of the marcher lordships (local rulers in Wales who were partly independent of the English crown) until 1536, when King Henry VIII created the county of Montgomeryshire and thus brought the area fully under the administration of the crown. From the 15th century the county was known for its manufacture of wool flannels and for its lead mining. The huge Powis Castle mansion, located outside Welshpool, is one of the best-known castles in Wales.