After the Romans abandoned Britain in 410 CE, the Brittanic tribes were left vulnerable to attack. Consequently, a period of migration into Britain by Germanic tribes began as they settled in the eastern regions and pushed the Britons west. In some places the Britons co-existed with the Germanic settlers, but Britons were generally subordinate to the newcomers. By around 600 CE a number of Anglo-Saxon kingdoms had been established in Sussex, Kent, East Anglia and parts of Yorkshire. Æthelberht, king of Kent, who ruled between c. 589–616, held the additional title of Bretwalda, which signified a level of power over the rest of the southern Anglo-Saxon Kingdoms. Æthelberht was also a key figure in the conversion of Anglo-Saxons to Chirstianity, helping convert Sæberht, the king of Essex. In Wessex, King Cealin took Cirencester, Gloucester and Bath, but his expansion westwards was halted by internal divisions within the Anglo-Saxon kingdoms. In the north, the kingdom of Northumberland, which comprised Bernicia and Deira, was a powerful entity in its own right.