A History of the Anglo-Saxons in England
The action of The Circle of Ceridwen Book One takes place in 871, when most of Britain was a series of small, warring Anglo-Saxon nations. The first recorded Viking attack on Britain was in 793, but incursions remained sporadic until 865, when they increased in frequency and severity.
By 871 four of the seven Anglo-Saxon nations had been overrun by the Danes. Ælfred, at the age of 22, was named King of Wessex, one of the remaining kingdoms, in that same year. After a series of battles (many of which he lost) he was able to stop the advancing Danes that Summer, thus laying the groundwork for his later consolidation and defence of the country.
If you can stand a bit more history, you might enjoy “Snoozed Through History Class?”