DNF
TL:DR--It's not good enough to waste time on. Even if you like the subject. It's just too poorly written to suffer through this novel--especially as the first book in a multi-book series.
If you're interested in Roman military history, and a lot of readers are, what better way to experience some of these events than through historical fiction, right?
That's what I thought as well. However, the writing had a lot of problems that threw me out of the flow of the book. There were too many huh?-what? moments that brought the narrative to a screeching halt. These moments happened so frequently that I decided against spending more time on the book--especially given that the book is the first in a multi-book series.
There are a lot of books in this series, so either the author/editor got better, or other readers are better able to slog through the poor writing.
Here are some of the problems that I ran into
* Inconsistency errors. Many times (in the 25% of the book I managed to get through) the protagonist will walk someplace (jog, etc.) and then ride his horse back. Wait? What? Where did the horse come from, was he leading it and then decided (without telling the reader) to mount it, was someone else holding it for him, did it follow him like a loyal dog? Nope, none of those make sense. This is just lazy/sloppy writing and a lack of editing. Sure, I could still figure out the story line, but in an ancient battle being on a horse, or not, makes a big difference (vision, speed, endurance, notice-ability, etc.).
* Modern language use. I'm not expecting the characters to speak ancient Latin, but I *do* expect the author to give me the flavor of ancient Roman life and how the characters would experience and interpret their world. When one of the characters raises a concern about a psycopathic killer, that's just lazy. Call the killer a madman, but don't use anachronistic psychiatric terms.
Also, the characters will use each chapter's Roman terms but they don't seem to act like ancient Romans. For example, scarcely a mention of gods or religions, which would've been mentioned fairly frequently I would think (even if only used for invective effect).
* Cardboard characters. The characters have very little character, they are bland and uninteresting. The primary characters are trope-driven, but not well-written good tropes.
Here's the tough but competent non-com that doesn't want to become an officer (ho boy, that's a new one O_o).
Or there's the ultra-competent commander that doesn't want to muck about in politics.
Or the masterful (but bland) general who will stop at nothing to get what he wants (even using his men as, gasp, political pawns). FFS Caesar, one of Roman history's most interesting figures, written as a bland cardboard cutout. Ugh.
You can find better elsewhere, don't waste your time or money on these.