The South Hams were formerly part of the Brythonic (Celtic) Kingdom of Dumnonia later reduced to the modern boundary at the River Tamar as Cornwall presumably during the tenth-century reign of Æthelstan. Post-Roman settlement on coastal promontory hillforts, such as Burgh Island, follows the established pattern of trading—of tin in particular—found across the western, so-called 'Celtic', Atlantic coastal regions. In the later Anglo-Saxon era, the South Hams was a feudal estate consisting of all of the land between the River Plym and River Dart and south of Dartmoor with the English Channel forming the southern boundary. There is some evidence that Cornish was spoken and understood in the area until the late Middle Ages.