Dumbarton, Strathclyde, stands on a volcanic rock, which has a longer recorded history as a stronghold than any other place in Britain; was the centre of the independent British kingdom of Strathclyde from the fifth century until 1018;
in medieval times Dumbarton was an important royal castle; the importance declined after Cromwell’s death in 1658, but continued as a garrison until WW2; most of its buildings have gone and little has survived from the old castle.see The History of Dumbarton
video (3 min – history and today – the Dumbarton Castle)
On UK map:
By MrPenguin20 – Own work, CC BY-SA 4.0, Link