Romanitas takes place in a world where the Roman Empire never fell. The Romans don't quite span the entire world (damn Nionians), but they're certainly doing their very best, and it makes for an entertaining sort of background - North America is pretty much a continent-wide DMZ!
While there's been an impressive amount of scientific progress (zippy trains, helicopters ("spiras"), television, a sort of telegraph/video-phone) a few of the familiar Roman institutions are still in place. That is, it is an Empire, complete with Emperor, and slavery is still thriving. Gladiatorial combat, crucifixition, togas, olives... all your standard Roman fare, but with the niceties of the 21st century. Imagine a world with both orgies and dental care.
The reader troops through the world from both the top and the bottom of the social hierarchy. Sulien and Una, two of the protagonists, are both slaves. They are siblings - abaonded by their mother and passed through a succession of miserable owners. Una disappears from the picture for most of her youth, a nice trick, as the reader's imagination can probably insert all the awful dangers that would an attractive young slave girl has to face while growing up.
Sulien has more ups and downs - he evidences a rare (supernatural) healing talent which earns the attention of Rome's leading physician. He's swept up from poverty and treated almost as a son - boundaries that become painfully obvious when he shacks up with the boss's daughter and soon finds himself accused (and convicted) of rape. Una reappears to save him, and the two hit the road. Escaped slaves (and convicted felons) have a short lifespan in the Empire, so they need to find safety and keep a low profile.
Marcus Novius, the book's other protagonist, also craves a low profile. Marcus is the Emperor's nephew, and the de facto heir to the Imperial Throne. At the start of Romanitas, Marcus is attending his parents' funeral. They've both died in an accident (a mysterious one) and the shell-shocked Marcus is thrown into the deep end of Imperial politics. The Emperor himself is more Claudian than Augustan - he's adamantly turning a blind eye to the shenanigans of his family. He has bigger and better things to ignore. Marcus soon realises that his life is at risk as well and he also runs away.
It doesn't take long for the three fugitives to unite in an uneasy alliance. Marcus is out for justice, Una for freedom and Sulien for comfort. If Marcus regains his stature and defeats his shadowy enemies, he can pardon Una and Sulien and maybe even do something about slavery as an institution. Sulien's had a taste of the good life and would like it back (plus, he's not so keen on being crucified). Una is more skeptical, but as jaded as she is, she's secretly optimistic enough to give it a shot. Plus, the inevitable romantic frisson begins to sparkle.
Normally, a fantasy is about the stableboy falling for the high princess and spending a trilogy proving himself worthy. In Romanitas, Marcus is the slightly-useless high princess and Una is the stableboy. Any hereditary nobility in her is purely the result of owners fiddling with possessions over the years. Impressively, they both know their romance is doomed and they act like real people. Marcus and Una do a bit of stealthy leering, a bit of agonizing and a lot of concentrating-on-the-fact-that-the-entire-Roman-Empire-is-out-to-kill-them. I applaud their priorities.
Romanitas' other strong point is an broader extension of the first. This is a character-driven novel. We learn a lot about our three heroes - not biographically, but emotionally. The bulk of Romanitas isn't solving the mystery (Ms. McDougall isn't shy about saying whoddunnit) or slaying enemies (rather charmingly, our protagonists lose every fight), it is about three lost people trying to come to terms with with world around them. They all know what they're running from, but they're not sure where they're going. Ostensibly, all three of them have a goal in mind - be it "far away", "home" or "anywhere but here", but as the panic wears off and they get a chance to think, they each begin to question themselves and and what they stand for. There are no sudden epiphanies, just three very real, very flawed people wrestling with their own values.
Ms. McDougall has clearly spent a huge amount of time imagining a lavish alternate history, and then she doesn't actually talk about it. In the six years since its first publication, this is the bit of Romanitas that has driven some readers completely batty. The setting is just that - the background for the story. Alt-Rome is plenty intriguing, but it isn't actually the point of the book. The characters aren't the only ones that need to be congratulated on their priorities.
Anyone that picks up Romanitas purely for the joy of quibbling with another person's speculatively historical "what ifs" will be disappointed - they'd be better off playing Civilization. Romanitas is character-driven sf. Despite the wildly-imaginative setting, this is the simple story of three very different people all trying to work out their problems like real human beings (that is, clumsily). There's the occasional murder and crucifixion, but that's almost beside the point. And the sequel is even better.