 Complete the following phrase:  In 870 Amlaíb and Ímar besieged Ail Cluaithe, or Dumbarton, the capital of Strathclyde; they captured the stronghold following a four-month siege, and returned to Dublin in 871 with a great deal of plunder. Later that year they stormed the fortress of Dún Sobairche or Dunseverick in County Antrim with the assistance of the Cenél nEógain, whose leader was the High King Áed Findliath. Before the end of 871, Amlaíb returned to Scotland to suppress a Norse uprising against his father Gofraid mac Ragnaill. During this time 871872, Ímar and Amlaíbs son Oistín Eysteinn plundered Ireland from east to west and from north to south. By 878 the Findgaill and Dubgaill were domit in many parts of Britain, controlling territories in Scotland, Strathclyde, Northumbria, East Anglia, Wales and Mercia. Their expansion was eventually halted by Alfred the Great of Wessex, whose victory in the 