In 870, the Britons of the Old North around the Firth of Clyde came under Viking attack as well. The fortress atop Alt Clut ("Rock of the Clyde," the Brythonic name for Dumbarton Rock, which had become the metonym for their kingdom) was besieged by the Viking kings Amlaíb and Ímar. After four months, its water supply failed, and the fortress fell. The Vikings are recorded to have transported a vast prey of British, Pictish, and English captives back to Ireland. These prisoners may have included the ruling family of Alt Clut including the king Arthgal ap Dyfnwal, who was slain the following year under uncertain circumstances. The fall of Alt Clut marked a watershed in the history of the realm. Afterwards, the capital of the restructured kingdom was relocated about 12 miles (20 km) up the River Clyde to the vicinity of Govan and Partick (within present-day Glasgow), and became known as the Kingdom of Strathclyde, which persisted as a major regional political player for another 150 years.