When a Roman legion fights some Gauls, they are magically placed in a different world. This world has actual magic, has never heard from their world, and the society and politics closely resemble the Byzantian empire. Turtledove clearly knows his history, and makes sure to show how the legion adapts to the different world, the different language, the different gods. He handles that part well.
I first stopped reading the book because it just wasn’t compelling. I couldn’t bring myself to care about the cardboard characters (most experience a lot of character growth by way of adding a second adjective to their personal inventory halfway through the book). I picked up the book again some time later, and dropped it again immediately when the overly heroic and clever and self-inserted protagonist saves his one female acquaintance from an ongoing rape and commences victory bonking. (She’s of course totally into it and into him and god why.) Just … no.