The Viking settlement of Dublin was established by the chieftain, Turgeis, who built a castle where Dublin Castle now stands. Their hold on Dublin was often tenuous, undermined by fighting between Dubgaill (‘fairhairs’) and Findgaill (‘darkhairs’) who were possibly Norwegian and Danish Viking factions. They were twice expelled before returning in force in 917. After fighting against Brian Boru at Clontarf (1014), their military power was broken; Viking Dublin persisted as purely a commercial enterprise. After converting to Christianity, Sitric Silkbeard had the original Christchurch built in 1030, and the city acquired a resident bishop. Excavations have revealed flood banks along the Liffey, backed by a stone wall (c. 1100), allowing the drainage of marshland, and the rapid expansion of the city eastward in the 12th century. The Vikings were finally driven from Dublin in 1171 by Anglo-Norman invaders who reinforced the city’s defences. It became the capital of Norman Ireland.