I don't often get into the fiction sub-genre known as "alternative history". It's sometimes lumped together with science fiction and fantasy (which don't belong together, either). The typical setup is a hypothetical change in the outcome of a crucial historical event, framed as the question, "what would have happened if...". Quite a few of these novels involve ancient Rome.
In the case of The Seven Hills, by John Maddox Roberts, the question asked is "what would have happened if Hannibal had defeated the Romans (218–203 BC) and driven them out of Italy". That part of the story and its aftermath, wherein the exiled Romans establish a new hegemony in the northern forests of Germania, was told in Hannibal's Children (2002). The Seven Hills takes it from there to imagine the Romans' triumphant return, 100 years later, to reclaim the seven hills of Rome and take revenge against Carthage.
Roberts wrote The Seven Hills before beginning his popular SPQR series (see previous post), and the immaturity of style is evident. Even more evident is something I'd never really thought about before - it's really hard to create good fictional characters and "historical" events. Historical fiction writers have an advantage, in that they can start with real historical characters and events, with all their unlikely-but-true human quirks known to posterity. When starting with the tabula rasa of a fictional character and/or event, however, all those complex, idiosyncratic, self-contradictory human traits have to be invented and - even more difficult - made believable to the reader.
Roberts faced that challenge in The Seven Hills - and lost. The fictional Romans, Carthaginians and others are not nearly as interesting as the historical Hannibal and his treacherous family, or Fabius and the rest of the inept Roman generals who opposed the Carthaginian invasion. Roberts' invented course of events lacks the kind of granular detail and unexpected shifts that make historical accounts so interesting.
Unless you're a big fan of Roman alternative history novels, I don't recommend The Seven Hills. The SPQR series is much better, and a big advance for Roberts as a writer.