With Vespasian, Narcissus rode away from the dusty chaos of the soldiers’ camp-building near the river bank. On the afternoon of this hot day, Narcissus was sweating as heavily as the horse beneath him. But as always it was a relief to get away from the army for a while; after another day on the march tens of thousands of men and their animals produced a tremendous stink.

They headed up to a scrap of higher ground, a ridge. Narcissus’s horse picked its way cautiously over chalky earth littered with flints, which Narcissus inspected curiously. He had seen almost identical terrain throughout northern Gaul. It was as if, he mused, Gaul and Britain were in reality a single landscape, severed by a strip of Ocean as a surgeon’s blade amputates a limb. It was an intriguing notion, but he had no idea how such huge changes in the structure of the earth could have come about. Perhaps Britain was a relic of Atlantis, he mused, or a bit of builder’s debris left over from primordial days when giants constructed the earth.

From the ridge they looked west, to the river, and the soldiers who swarmed near its bank. An overnight fortress had been set out above the ford, constructed in a few hours despite the men’s usual grumbling after a day of laden marching - but soldiers always complained, Vespasian said. The fort’s rectangular formation was marked out by a ditch and a low bank topped by a palisade of wooden stakes, hastily lopped from a scrap of woodland nearby. In the interior the legionaries’ leather tents were being set up in their usual rows. Already cooking smells curled up from a dozen fires, and the digging of latrines was itself a minor industry.

And when Narcissus looked further west, across the shining body of the river, he could see another force massed on the opposite bank. They were the Britons, here to oppose the Roman advance. The Britons, lacking any of the obvious discipline of the Roman troops, looked more like an urban mob, Narcissus thought idly, transplanted from Milan or Rome. Some of them seemed to be enacting some kind of ritual at the edge of the water. Narcissus could swear that they were breaking cups and plates, even weapons, and dumping the remains in the water. Was the barbarian mind really so bewildered that it imagined it was a good idea to smash your weapons and dump them in the river on the eve of battle?

But, disorganised and incomprehensible as they were, there were tens of thousands of them, Narcissus saw uneasily, perhaps even outnumbering the Roman forces. And at the rear of the crowd congregated by the river he saw horses drawing small, rapid, two-wheeled carts to and fro. They were the famous chariots of which Caesar had written so eloquently, rehearsing for war.