Dumbarton Castle has the longest recorded history of any stronghold in Scotland. It overlooks the Scottish town of Dumbarton, and sits on a plug of volcanic basalt known as Dumbarton Rock which is 240ft high.HistoryIron AgeAt least as far back as the Iron Age, this has been the site of a strategically important settlement. Its early residents were known to have traded with the Romans. The presence of a settlement is first recorded in a letter Saint Patrick wrote to King Ceretic of Alt Clut in the late 5th century.Early Medieval EraDavid Nash Ford has proposed that Dumbarton was the Cair Brithon ("Fort of the Britons") listed by Nennius among the 28 cities of Sub-Roman Britain. From the fifth century until the ninth, the castle was the centre of the independent Brythonic Kingdom of Strathclyde. Alt Clut or Alcluith, the Brythonic name for Dumbarton Rock, became a metonym for kingdom. The king of Dumbarton in about AD 570 was Riderch Hael, who features in Welsh and Latin works.