OK, a bit offended by being labeled a 'purist' by the author in the afterword, but here it goes anyway.
This would be better set as 'an alternate world ' fantasy because of the liberties taken by the author. Rus/vikings half a millennium ahead of their time, Viking dueling rituals a millennium before the first accounts of them, continuing confusion over 'barbarian' tribes.
....and of course our invented hero and his ahistorical Chinese kung fu monk sidekick.
But a fun rollicking yarn, pity it is labeled historical fiction.