Dumbarton Castle (Scottish Gaelic: Dùn Breatainn, pronounced [t̪unˈpɾʲɛʰt̪ɪɲ]) has the longest recorded history of any stronghold in Great Britain. It overlooks the Scottish town of Dumbarton, and sits on a plug of volcanic basalt known as Dumbarton Rock which is 240 feet (73 m) high.
At 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 (or Clyde Rock) in the late 5th century. From the fifth century until the ninth, the castle was the centre of the independent British Kingdom of Strathclyde. The King of the Britons of Dumbarton in about AD 570 was Riderch Hael, who features in Norse legends. During his reign Merlin was said to have stayed at Alt Clut. In 756 the first (and second) losses of Dumbarton Rock were recorded. A joint force of Picts and Northumbrians captured Alcluith after a siege, only to lose it again a few days later. By 870 Dumbarton Rock was home to a tightly packed British settlement, which served as a fortress and as the capital of Alt Clut.