Roman military withdrawals left Britain open to invasion by pagan, seafaring warriors from north-western continental Europe, chiefly the Saxons, Angles, Jutes and Frisians who had long raided the coasts of the Roman province. These groups then began to settle in increasing numbers over the course of the fifth and sixth centuries, initially in the eastern part of the country. Their advance was contained for some decades after the Britons' victory at the Battle of Mount Badon, but subsequently resumed, overrunning the fertile lowlands of Britain and reducing the area under Brittonic control to a series of separate enclaves in the more rugged country to the west by the end of the 6th century. Contemporary texts describing this period are extremely scarce, giving rise to its description as a Dark Age. The nature and progression of the Anglo-Saxon settlement of Britain is consequently subject to considerable disagreement; the emerging consensus is that it occurred on a large scale in the south and east but was less substantial to the north and west, where Celtic languages continued to be spoken even in areas under Anglo-Saxon control. Roman-dominated Christianity had, in general, disappeared from the conquered territories, but was reintroduced by missionaries from Rome led by Augustine from 597 onwards. Disputes between the Roman- and Celtic-dominated forms of Christianity ended in victory for the Roman tradition at the Council of Whitby (664), which was ostensibly about tonsures (clerical haircuts) and the date of Easter, but more significantly, about the differences in Roman and Celtic forms of authority, theology, and practice.