The Strathclyde Britons were defeated by Ivar the Boneless and the Vikings after a four-month siege of their chief fortress, Dumbarton Rock, in 870. The Vikings took many of the Britons to Dublin as slaves, though a few survivors fled to their kinsmen in Wales. The battle destroyed the kingdom of Strathclyde, which had been one of the three principal kingdoms of Scotland: it is possible that Constantine I of Scotland supported the Vikings.