The angel and the sword

"It is the waning days of the ninth century. The vast dominion won by the great Emperor Charlemagne has been torn apart by his heirs and successors. In Paris, once the heart of that realm, the Emperor's irresolute grandson, Charles the Bald, battles desperately to preserve his kingdom from the ravages of Viking raiders. If the city falls, its people will perish, and along with them the precious remnants of culture and learning rescued from the ashes of Rome." -- Jacket.