In this scenario, you are the Viking Civ on an island north of a mainland looking like the northeast coast of Scotland or England. Your enemies are the Celts (Green) and Britons (Red) on the mainland.
There is no Story. Conceivably, the Vikings are "avenging" the Celts by attacking the Anglo-Saxon tribes, represented ironically by the Celts and Britons, who were a Celtic tribe.
The Anglo-Saxons were a Germanic group, like the Goths, Teutons, Franks, and Vikings were.