Kingdom of Strathclyde
From the 9th to the 12th century Viking/Norse-Gael Dublin in particular was a major slave trading center which led to an increase in slavery. In 870, Vikings, most likely led by Olaf the White and Ivar the Boneless, besieged and captured the stronghold of Dumbarton Castle ("Alt Clut"), the capital of the Kingdom of Strathclyde in Scotland, and the next year took most of the site's inhabitants to the Dublin slave markets.