Mercian was a dialect spoken in the Anglian kingdom of Mercia (roughly speaking the Midlands of England, an area in which four kingdoms had been united under one monarchy). Together with Northumbrian, it was one of the two Anglian dialects. The other two dialects of Old English were Kentish and West Saxon. Each of those dialects was associated with an independent kingdom on the island. Of these, all of Northumbria and most of Mercia were overrun by the Vikings during the 9th century. Part of Mercia and all of Kent were successfully defended but were then integrated into the Kingdom of Wessex. Because of the centralisation of power and the Viking invasions, there is little to no salvaged written evidence for the development of non-Wessex dialects after Alfred the Great's unification, until the Middle English period.