Vikings traded with the Gaelic, Pictish, Brythonic and Saxon kingdoms in between raiding them for slaves. The Saxon slave traders sometimes worked in league with Norse traders often selling Britons to the Irish. In 870, Vikings besieged and captured the stronghold of Alt Clut (the capital of the Kingdom of Strathclyde) and in 871 took most of the site's inhabitants, most likely by Olaf the White and Ivar the Boneless, to the Dublin slave markets. Maredudd ab Owain (d. 999) paid a large ransom for 2,000 Welsh slaves, which demonstrates the large-scale slave raiding upon the British Isles.