My general opinion of Turtledove's output is that he does better when he's doing stand-alone books than when he's into the second or later volume in a series, and he does better with further-back alternate-history turning points than he does with 19th and 20th century turning points. (Down in the Bottomlands, with a turning point about 6 million years ago, is one of his best ever.) This book bears out that general opinion. It may turn out to be the first in a new series (more about that later) but for now, it appears to be one of a kind.
The turning point for this alternate history appears to be about 12,000 years ago - when, in our world, the last ice age started to end, and the glaciers receded to above the Arctic Circle. I can't be entirely sure how far forward after that turning point we are; civilization seems to have reached a late-Middle-Ages stage, with bows and arrows as weapons, but no crossbows or firearms, and horses being ridden but no mention of stirrups. (Those who have read Jared Diamond's "Guns, Germs and Steel" know that the invention of the stirrup may have been one of the biggest turning points in the history of warfare.) There's writing, but not everybody is literate; there are walled cities, but also nomadic tribes living at a level barely above Stone Age. The overall term for the clans of nomads is "Bizogot" and I don't know whether Turtledove intended us to assume that because this sounds a little like "Visigoth," the proto-Indo-European language that developed into the Western languages we know in our world developed in some similar manner in this fictional world. He may just have meant it to give us rough associations of "barbarians prepared to invade the empire."
As it turns out, however, the barbarians and the Empire must make common cause, due to a new danger from the other side of the glacier, because a gap has opened between east and west. Here is where the book starts to hit weaker moments: there are some logical inconsistancies posited in the lack of knowledge our protagonists have about the newly discovered others (whereas they know about people to the south of the Empire, who presumably have been able to travel further around the world unimpeded by glaciers), and the new people, who style themselves "the Rulers," are portrayed in a strictly cardboard bad-guy fashion. Also, as one previous reviewer mentioned, at this point we get competing wizards performing magic, with no attempt at scientific explanation, which for many people would take the book into the realm of fantasy and per force remove it from alternate history. However, if one is willing to allow one's suspension of disbelief to cover the wizards - there may, after all, be things we do not yet know - then this is not a sticking point. On the other hand, my own perspective on suspension of disbelief gave me a few bad moments at what seemed to me the improbable resolution of the romance between Hamnet and Liv.
The book ends in an open-ended fashion, which is why I suspect there may be a sequel planned, and I would worry about that a little, because if Turtledove were to follow his usual track record, the sequel would be almost entirely about battle plans and the development of new weapons, with very little attention given to any character development, or to any plot outside of war and more war. However, that hasn't happened yet; perhaps I'm worried for nothing. And then, there are many people who seem to LIKE those interminable war series, so you may well have something to look forward to!
Oh yes, one more thing: our protagonist and one of his companions are awful punsters. Read these puns at your own risk!