Dumbarton Castle Dumbarton Castle ( , ) 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 high . History . Iron Age . 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 in the late 5th century . Early Medieval Era . From the fifth century until the ninth , the castle was the centre of the independent British Kingdom of Strathclyde . Alt Clut or Alcluith ( , , literally " Clyde Rock " ) , the Brythonic name for Dumbarton Rock , became a metonym for kingdom . The king of the Britons of Dumbarton in about AD 570 was Riderch Hael , who features in Welsh and Latin works . During his reign Merlin was said to have stayed at Alt Clut . The medieval " Scalacronica " of Sir Thomas Grey records the legend that " Arthur left Hoël of Little Britain his nephew sick at Alcluit in Scotland . " Hoël made a full recovery , but was besieged in the castle by the Scots and Picts . The story first appeared in Geoffrey of Monmouth ' s " Historia Regum Britanniae " . Amongst lists of three things , in the triads of the " Red Book of Hergest " , the third " Unrestrained Ravaging " was Aeddan Fradog ( the Wily , perhaps Áedán mac Gabráin ) , coming to the court of Rhydderch the Generous at Alclud , who left neither food nor drink nor beast alive . This battle also appears in stories of Myrddin Wyllt , the Merlin of Geoffrey of Monmouth ' s " Vita Merlini " , perhaps conflated with the battle of Arfderydd , located as Arthuret by some authors . 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 . The Vikings laid siege to Dumbarton for four months , eventually defeating the inhabitants when they cut off their water supply . The Norse king Olaf returned to the Viking city of Dublin in 871 , with two hundred ships full of slaves and looted treasures . Olaf came to an agreement with Constantine I of Scotland , and Artgal of Alt Clut . Strathclyde ' s independence may have come to an end with the death of Owen the Bald , when the dynasty of Kenneth mac Alpin began to rule the region . Medieval Era . In medieval Scotland , Dumbarton ( " Dùn Breatainn " , which means " the fortress of the Britons " ) was an important royal castle . It sheltered David II and his young wife , Joan of The Tower after the Scottish defeat at Halidon Hill in 1333 . In 1425 the castle was attacked by James the Fat , youngest son of Murdoch Stewart , Duke of Albany , who had been imprisoned by King James I of Scotland on charges of treason . James the Fat became a rallying point for enemies of the King , and raised a large rebellion against the crown . He marched on the town of Dumbarton and burned it , but was unable to take the castle , whose defender John Colquhoun successfully held out against James ' men . Patrick Hepburn , 1st Earl of Bothwell , was Captain of Dumbarton castle on 1 April 1495 . Sixteenth century . During the war of the Rough Wooing , the castle was briefly occupied against the Scottish Government of Regent Arran by the Bishop of Caithness , Robert Stewart , a brother of Matthew Stewart , 4th Earl of Lennox , who came from England with the support of Henry VIII . He had sailed from Chester with around 20 followers in May 1546 in the " Katherine Goodman " and a pinnace . Having borrowed the artillery of the Earl of Argyle , Arran successfully besieged the castle , which surrendered after 20 days . The chronicle historian John Lesley wrote that the Captain and the Bishop surrendered the castle to Arran and were rewarded , after negotiation by the Earl of Huntly . The siege at Dumbarton delayed Arran ' s action at the siege of St Andrews Castle on the east coast of Scotland . In 1548 , after the Battle of Pinkie , east of Edinburgh , the infant Mary , Queen of Scots was kept at the castle for several months before her removal to France for safety , where she was soon betrothed to the young dauphin Francis . In October 1570 during the Marian civil war , John Fleming , 5th Lord Fleming fortified the castle for Mary against the supporters of James VI of Scotland with stones he obtained by demolishing churches and houses in Dumbarton and Cardross . His defence of Dumbarton for Mary was satirized in a ballad printed by Robert Lekprevik in May 1570 ; " The tressoun of Dumbertane . " Attributed to Robert Sempill , the ballad describes Fleming ' s failed ambush of Sir William Drury . The castle was captured by the forces of the Regent ( Matthew Stewart , 4th Earl of Lennox ) led by Thomas Crawford of Jordanhill and John Cunningham of Drumquhassle in the early hours of 2 April 1571 , who used ladders to scale the rock and surprise the garrison . Seventeenth century . The castle ' s importance declined after Oliver Cromwell ' s death in 1658 . Due to threats posed by Jacobites and the French in the eighteenth century , Britain built new structures and defences there and continued to garrison the castle until World War II . The castle today . Today all visible traces of the Dark - Age Alt Clut , its buildings and defences , have gone . Not much survives from the medieval castle : the 14th - century Portcullis Arch , the foundations of the Wallace Tower , and what may be the foundations of the White Tower . There is a 16th - century guard house , which includes a face which according to legend is " Fause Menteith " , who betrayed William Wallace . Most of the existing structures were built in the 18th century , including the Governor ' s House , built for John Kennedy , 8th Earl of Cassilis , and fortifications which demonstrated the struggle by military engineers to adapt an intractable site to contemporary defensive needs . The splendid views from the twin summits of the White Tower Crag and the Beak remind us why this rocky outcrop was chosen as ' the fortress of the Britons ' centuries ago . The castle is open on a daily basis during the summer season and Saturday - Wednesday in the winter . Visitors must climb the 557 steps to see the White Tower Crag and other features . Dumbarton Rock is in state ownership and is legally protected by the Scottish Government as a Scheduled Ancient Monument , to conserve it for future generations . Activities such as rock climbing are forbidden ; any change or damage caused is considered a criminal offence . External links .