Boudica: Dreaming the Eagle, by Manda Scott <br></br>The Druid King, by Norman Spinrad <br></br>The Gates of Rome, by Conn Iggulden <br></br>A Song for Nero, by Thomas Holt

There is a room in the Capitoline Museum that is full of the heads of emperors. You walk in from the heat of a Roman morning, and there they are, 40 pairs of marble eyes, sneering coolly across the ages. The scary thing is how many of them look just as we expect – Caligula has calculating, brutal eyes; Heliogabalus is so achingly pretty, and mildly deranged. They are familiar because they haunt Europe's imagination; they are our story of how absolute power corrodes the soul but gets to wear clean, white linen or golden armour along the way.

To be a European is to be descended from the people whom they beat into submission or who helped kick them when they were eventually down. It is to see the Romans as the builders of the continent's first long-distance roads and the people who tortured their enemies to death on crosses; it is to marvel at a state that endured for a half-millennium and to be appalled at its corruption and fall. Much of European history and culture comes from this split attitude: we want to be like the Romans and to be horribly warned against being like them.

And to watch America, as its democracy falters and the ambitions of its rulers grow is to think of how, once before, a Republic lost its soul and became an Empire. To spare the conquered, and put down the proud, sounds like a very good idea. Somehow, though, it always ends in blood, and fire, and screaming. Maximus in Ridley Scott's Gladiator sums it up when he tells his men: "Unleash hell." And they do.

The best of the current crop of novels about Rome, its empire and its victims, is the most sensible and level-headed, in spite of its touches of infatuation with things Celtic. Manda Scott's earlier books are veterinary thrillers. What draws her to the ancient Britons in Dreaming the Eagle, the first of a sequence of novels about Boudica, is her sense of them as a people whose wealth and civilisation lay, in part, in their relationship with animals. This sense is never sentimental and always tough-minded. The hounds and horses that preoccupy young Breaca and her brother Ban are real animals that snort and get sick, at once obstinate individuals and useful extensions of human will.

For her Romans, animals are only tools, but then, so are people. This is an interesting historical novel because it deals with a clash of two very different senses of the sacred. Scott is almost as good on the authoritarian, rational piety of the best of the Roman invaders as she is on the wild, intense spirituality of her Druids and women warriors. Significantly, the villain for her is not any of her Romans, not even the demented and capricious Caligula himself, so much as a British traitor: Amminios, one of the sons of Cymbeline, to whom nothing is sacred, and animals and men are just pieces to move around the chequerboard of his mind.

It helps that, in a sometimes cluttered way, Scott has an intense visual sense. This redeems a writing style that is at best clear and precise, but never more interesting. She has a passion that is lacking, most of the time, from Norman Spinrad's highly professional The Druid King, which does something similar for Vercingetorix and his struggle with Caesar.

The conceit here is that the vision-haunted hero gradually realises that his destiny is to lose for the greater good – Gaul has to go down in order for it to become modern France, a millennium or two later. Everything is for the best in this best of all possible worlds; charmingly treacherous Caesar is a Good Thing even if he is a Bad Man.

This is a vision of history as ruthless process that Spinrad has got away with in his SF novels; if it fails to work here, it is partly because, when we know the outcome of the story, telling us that everything is inevitable is just an inevitability too far. When we learn that Vercingetorix has visions of the future, we guess, because the author has a sweet tooth for irony and shadowing, that his great enemy's fits give him the same powers. When Vercingetorix acquires a sworn Amazon sister-in-arms and a Roman-educated beloved, you know, because this is a Spinrad novel, that he will end up having sex with them both, and guess, correctly, that it will probably be at the same time. Spinrad knows his way round Caesar's histories and rightly positions them as brilliant spin doctoring, but there is something airless about this, especially when he's at his most clever.

It is never as airless, however, as Conn Iggulden's The Gates of Rome, the first of a sequence about the life of Julius Caesar. Again, much is made here of historical inevitability; young Gaius is brought up to fight and think by a politician father, and a couple of former gladiators, as is his young friend Marcus, whose identity we are not meant to guess.

Iggulden takes us through a slave revolt and a simplified account of the brutal coup and counter-coup of Marius and Sulla, the rival generals who weakened the Republic to a point where Caesar and his rivals could destroy it. It does not inspire confidence in a historical novelist when he explains that he has drastically changed the facts for convenience. And, yes, it turns out that Marcus is the future Brutus; imagine the irony!

Thomas Holt's A Song for Nero has a lot of fun with the worst Roman of them all, as he wanders posthumously around an Empire that curses his name with the petty con-man brother of the double who died in his place. Having come up with this splendid picaresque idea, Holt then proceeds to weaken it rather with a transparently silly McGuffin – all sorts of gangsters and corrupt officials know of the Emperor's survival and are keen to extract from him knowledge of a treasure.

Those of us who admire the thrillers of Lindsey Davis already know how easily the mean streets of Los Angeles noir map on to Vespasian's Rome. Holt is at his best when he is just enjoying a complicated caper in which the extreme moral ambiguity of the narrator's best friend is just one of the jokes. At times, though, the jokes go on to automatic pilot in the way we are used to in Holt's comic fantasies.

The weakest of these four books is the one that is only about the grandeur and gore of Rome. The point of historical novels should always be that they say something about us as well as about the Other they describe – we should always be able to see our own face and our civilisation's own crimes, looking at us out of those cold, dead eyes.