Ebraucus also founded the city of Alclud over in Albany; and the castle of Mount Agned, which is now called the Maidens’ Castle and the Dolorous Mountain. [II.7]

ALCLUD — Dumbarton in Scotland — is said by Geoffrey to have been founded by Ebraucus, who also founded the city of YORK. Tatlock writes that Dumbarton was one of the most ancient towns of Scotland (p. 14), and as Alclud it is certainly prominent in the affairs of the north as Geoffrey reports them. It is the place where Elidurus the Dutiful fools his people into swearing allegiance again to his deposed brother Archgallo:

When Elidurus had been King for some five years, he came upon his deposed brother one day when he was hunting in the Forest of Calaterium.... The moment he saw him, Elidurus ran up to him, embraced him and kissed him repeatedly. When he had spent some time lamenting the misery to which Archgallo was reduced, Elidurus took him to one of his cities called Alclud and there he hid him in his own bedroom. He then pretended to be ill and sent messengers through the kingdom for a whole year to request the princes under his jurisdiction to come to visit him. They all assembled in the town where he lay and he ordered them each in turn to come into his bedroom without making a sound... [III.17]

The town features as a base and focus for the struggles of both Arthur and his father with the Picts and the Scots:

After this victory Uther went to the town called Alclud, and while there he settled the affairs of that region and restored peace everywhere. [VIII.19]

Arthur was labouring under very considerable difficulties, for he had left behind in the city of Alclud his cousin Hoel, who was seriously ill. He finally reached the country of Somerset and approached the siege. [IX.3]

As soon as King Arthur had gained the upper hand, he ordered Cador, the Duke of Cornwall, to pursue the Saxons, while he himself hurried off in the direction of Albany. It had reached his ears that the Scots and the Picts had besieged his nephew Hoel in the town of Alclud, where, as I have explained already, Arthur had left him because of his poor health. Arthur therefore hastened to his nephew's assistance, for he was afraid that Hoel might be captured by the barbarians.... Once peace was restored... Cador set out for Alclud. Arthur had already freed the town from the harassing attentions of the barbarians. He now led his army to Moray, where the Scots and the Picts were under siege. [IX.5-6]

And it is one of several bishoprics settled after Arthur's victories (although, as Tatlock points out, it never was a bishopric, p. 16):

The bishopric of Silchester was given to Maugannius, that of Winchester to Diwanius and that of Alclud to Eledenius. [IX.15]

The name as it appears in Geoffrey reflects Welsh usage; Tatlock points to its appearance in Welsh poetry, p. 15. It derives from Ail Chluaidh, or Rock on the Clyde: the prominent rock at the mouth of the River Leven, now known as Dumbarton Rock, was a focal point in the ancient kingdom of Strathclyde. The name Dumbarton derives from Dun Breatann or Breatuin, meaning “the hill or fort of the Britons.” Dumbarton Rock is popular with climbers: see, for example, its page at Scottish Climbs. The BBC’s Scottish History site includes a Dumbarton Rock factsheet. There is also a site for Dumbarton Castle.