This is volume 5 of the adventures of Ballista (or Dernhelm), the son of an Angle warlord in Germania who grew up as a hostage of the Romans, was brought up at the imperial court and became a Roman general specialized in siege warfare. In this volume, after having been sent on a dangerous mission in the Caucasus, he is now sent with his "familia" on another dangerous mission to the Goths and Heruls living North of the Black Sea.
Apart from Harry Sidebottom's usual research, description of characters and writing style - all excellent - this book has quite a few other things going for it. The other strong point, of course, is that, as in his four previous books, and as another reviewer has mentioned already, the author makes you learn a lot in a rather entertaining way. However, none of these points are specific to this book. There are generic and can apply just as well to any of his four previous books.
The same reviewer thought it useful to warn readers against potential criticism that this book might seem slow paced at time. It does seem slow, at one point, but, far from being a criticism, this is perfectly suitable and it even seems to be deliberate since it conveys very well the impression of an endless trek as the embassy crosses the steppe and seeks to reach the Heruls' camp.
Another point is Sidebottom's choice of the Heruls, of which we know very little, and what we know mainly comes from Procopius writing almost 300 years after the time when the book's story is supposed to take place. This, of course, allows the author more room for invention. To my limited knowledge at last, there is nothing in the sources stating the Heruls practiced cranial deformation (unlike the Burgonds, who did, at least for their nobles) and Heruls were Germanic and not Huns, although their description seems to match the latter in some aspects, but then there is nothing that explicitly excludes it, so why not?
Other bits are fascinating since they offer a glimpse of steppe politics: how Rome (and then Byzantium) strived to keep the various tribes fighting each other and tried to avoid having any confederation becoming too powerful through bribes and alliances. It also gives a rather vivid description of what a steppe cavalry battle could have looked like and felt. Then there is another interesting twist in the story: among the embassy, there happens to be a psychopath that would nowadays qualify as a serial killer so that this book, in addition to being a historical novel, will also have you trying to guess "who dune it" (by the way, there are plenty of "red herrings" and I got it wrong!).
There is one thing, however, that makes this book worth a strong four stars rather than five: it does not stand alone very well. Although, to be fair, there is quite a few explanations as to who the main characters are and what has happened in previous episodes, it is definitely preferable to read the five books one after the other.
Anyway, I much preferred this one (I found Caspian Gates a bit "tepid"). It also seems that Harry Sidebottom has decided to treat us to a tour of the Roman Empire's neighbors and borders in the 3rd century AD since book 6 will be called the Amber Road, coming after the Persians, the Caucasus, the Goths and the nomads North of the Black Sea. Another treat in preparation...