This is one of Turtledove’s early books, dating back to 1987, and one of
the first of his works to appear under his own name; prior to this, he’d
written mostly short fiction under the name Eric G. Iverson. These days
Turtledove’s best known for his novels of alternate history;
this is something similar, yet not quite the same.

Marcus Scaurus is the commander of a Roman legion fighting under Julius
Caesar in Gaul. During a battle with the leader of a Gaulish troop, druid
magic sends Scaurus and his legion (and his opponent) to another world, a
world almost impossibly strange, to a place called the Empire of Videssos.
Videssos is what we’d think of as a proper empire, ruled by an emperor
and controlling vast regions; the Roman empire Scaurus knew was still
ruled by the Roman Senate in the name of the Senate and People of Rome.
Moreover, Videssos is an empire of long standing, and its court protocols
and politics are singularly convolute. The people of Videssos and most
of the surrounding countries worship a single god named Phos, though in
slightly different ways from country to country, which leads to a fair
amount of strife; Scaurus and his men naturally worship the gods of Rome.

Has the penny dropped yet? That’s right, The Misplaced Legion
is really about what the Byzantine Empire, an empire which still called
itself Roman, would look like to a Roman of Caesar’s day. And the
answer, like nothing on earth. Oh, Turtledove’s dressed it up a bit.
Persia is to the west of Constantinople–excuse me, Videssos the
City–instead of to the east; there’s no analogue of Rome, Videssos the
City has always been the capital of Videssos the Empire; the religion is
roughly Zoroastrian instead of Christian; all the names have been
changed, except they mostly sound like Greek anyway. Oh, and there’s
magic; and since the dominant religion is Zoroastrian with the Videssians
as the followers of Ahura Mazda, naturally the bad guys are wicked as all
get out and worship Ahura Mazda’s opposite, the loathsome Ahriman.
Though of course, they call him Skotos instead, just like they call Ahura
Mazda Phos. This is called poetic license, I suppose.

Anyway, it’s an OK book, if not quite as good as I remembered; there are
three more in the immediate series, plus some spin-offs, and no doubt
I’ll get to all of them again in time.