A very entertaining history book, or a rather mediocre novel? Well, both at once, paradoxically.
First, to deal with the book as a novel. Sidebottom is a historian, and he never lets you forget it. That's no bad thing of course, if you're looking at this as a history textbook. But from the point of view of a novel, it's too full of 'laundry lists' - detailed descriptions of cultural artefacts and customs which in no way contribute to the plot; as for names, Sidebottom positively drowns you in them. So many, and so similar. This is the problem when you deal with history, you have no option but to deal the hand you've got; if that means not coming up with characters who are sufficiently different from each other to make the action easier to follow, then that's factual narrative for you.
The other problem is that Sidebottom lets you see glimpses into the mind, the inner person, of people who actually existed. And therein lies the biggest problem : because these are historical characters, but only documented in their own times in the ancient fashion of reputation, exaggeration and anecdotal evidence, they are essentially unknown as real people. That's not an issue if your name is Hilary Mantel, but Sidebottom would be first to admit that he is no Mantel. Too much 'telling' and not enough 'showing', no real imaginative insights into the emotions and motivations of the main characters. So as a historical novel, this is not a great coup.
However, if you turn it around and say : here is a most exciting and eventful period of Roman history, how do you make it come alive? Turning it into a novel form while presenting actual history (leading statesmen, supporting politicians, military leaders, Emperors and Empresses and their children, battles, politics, intrigues, strategy etc) is a very good and effective way to present, and therefore teach, history. On that aspect, Sidebottom has delivered a triumph.
And make no mistake - he understands how to keep the tension going, how to create a 'page turner'. No matter that it is really speaking a history textbook, it's a whole lot more fun than reading Wikipedia.