The name "Dumbarton" comes from the Gaelic Dun Breatann, Fort of the Britons. Dumbarton Rock is mentioned by Monmouth as the site of one of Arthur's (the King of the Britons) northern campaigns against the Picts and Scots. It is a large rocky peninsula with two peaks. The smaller peak at Dumbarton Rock was the one occupied in the Dark Ages, but the other peak has larger, recent ruins. The site overlooks the River Clyde, and you can almost see all the way to the mouth of the River Clyde towards Greenock on the opposite bank.
Dumbarton Rock has a long history of occupation and the rock was settled before the town of Dumbarton itself. This region controlled the whole west of Scotland. Dumbarton Rock was a stronghold in the 5th and 6th centuries, the time of Arthur.
The association of Dumbarton Rock with Arthur is less certain, though the place is mentioned in passing in several of the legends. The legend also associates Dumbarton Rock with Merlin, who stayed for a time in the court of Rhydderych Hael, a ruler of Dumbarton.