They stayed a couple of nights at a place called Eburacum. This was a city of massive walls and towers strung along a riverfront, looking down on the civilian town that huddled around it. A huge building loomed out of the centre of the town, visible for miles around. It was the Roman military headquarters, Tarcho said.

Founded as a legionary headquarters Eburacum had always been an important place. One emperor had died here: Severus, a century ago, after his campaigns in the Highlands, and after making Eburacum capital of one of his two British provinces. Since then the fortress and its walls had been rebuilt, massively. And another emperor had been created here, in Constantine, who had been proclaimed in that imposing headquarters building. Now Eburacum was the base of the military commander of the north, the Duke of the Britains.

But Thalius didn't like the place. 'With its aloofness and arrogance and monumental military architecture,' he said, 'it prefigures in stone the haughtiness of the absolute monarchy of the future.' Audax didn't think even Tarcho knew what he was talking about.

Travelling further north still they passed through more hilly country. The sky was huge and full of immense clouds. Somehow Audax found this wilder, more rugged landscape less intimidating than the crowded hills of the south. Thalius gently pointed out that this country was Brigantia, Audax's home. But none of Audax's ancestors had seen home for generations.

Tarcho grew more animated. He pointed out forts and camps and watchtowers that were part of a 'deep defence system', he said, reaching far back into the countryside south of the line of the Wall itself. And the land was studded by big blocks of greenery at the crowns of the hills and in the valleys. They were managed forests, planted especially to provide the Wall with timber for its baths and ovens. While the Wall was here to defend the country from the savages in the north, the country had to feed the Wall. Audax began to think of the Wall as a great ravenous beast, sucking the blood from a cowering land.

They arrived at last at a place called Banna, where there was a fort.

Before it reached the fort itself the road snaked through a patch of farmland owned by the fort-the 'soldiers' meadow', Tarcho called it-and the party crossed over a ditch clogged with weeds and stinking rubbish.

Then they passed through a kind of town, sprawling east and west along the road outside the fort walls. The roads were more like sheep tracks than Roman roads. The place was noisy, smelly, crowded. Some of the buildings were quite smart and built to square plans, but the rest were just shacks. Many of them had open fronts, and Audax peered into shops where metal was worked or cuts of meat were piled high. There were soldiers, dressed in military belts or bits of armour like Tarcho's. But there were plenty of women, and children ran everywhere, getting in the way of the horses. Audax liked it better than Camulodunum. It seemed a cheerful place. But Tarcho hurried him past the soldiers' taverns, gambling dens and brothels.

At last they approached a stone wall. This was the fort itself. The buildings of the scrubby town outside lapped right up to the wall. At the fort gate they had to pay a charge, and the carriage was searched for weapons.

Inside the fort Audax was overwhelmed by a stink of blood and smoke and piss. Tarcho told him it was always like this; the soldiers used their own urine to cure leather for their armour and harness gear. Though Aurelia and Cornelius pressed bits of perfumed cloth to their noses, Tarcho opened his chest and sniffed in the foul air through his big, black, snot-crusted nostrils. 'Home! Nothing like it.'

The buildings, of stone, mudbrick and wood, were a bit more orderly than outside, the narrow cobbled streets between them straighter. But you could see the buildings were old and much repaired. Audax thought two big buildings with two storeys and sparkling tiled roofs must be palaces. Tarcho said they were granaries, where the soldiers stored enough grain to feed them for weeks, in case the barbarians ever attacked. There were more soldiers, including a few who lounged at their posts on the walls. The troops here were a thousand-strong cohort of Dacian origin, Tarcho said, called 'Hadrian's Own'. But nowadays most of the soldiers, locally recruited, were British, not Dacian.

Aurelia, her cloak over her arm to keep it off the muddy ground, looked around at the shabby fort with disdain. 'So this is what has become of the mighty Roman legions!'

'There were never any legions posted here, madam,' Tarcho said. 'In fact strictly speaking there are no more legions nowadays…'

She shuddered. 'By Jupiter I wish I'd never come here. If this is all that stands between me and the barbarian hordes of the Highlands I'll never sleep soundly again.'

The party split up. Thalius, Aurelia and Cornelius were taken to the fort commander's quarters, a grand old stone building. Tarcho took Audax to a much smaller house of mudbrick and thatch, one of a block. The house belonged to a soldier, an old family friend of Tarcho's, and it was no barracks, as in former times, but a home. Tarcho's friend lived here with his wife, two young sons and a whole pack of eager dogs. Audax didn't know what to make of the noisy bedlam, and the dogs, used to control the slaves in the mine, terrified him. But Tarcho had a quiet word with the soldier's wife, and she made a fuss of Audax and fed him bread and beef, and Tarcho showed him her husband's curving Dacian sword, a falx, and he began to feel better.

That night, in a small cot in the corner of a room he and Tarcho shared with the soldiers' sons, Audax slept well. He felt safe, encased by the walls of the house, and then by the walls of the fort, all of it watched over by soldiers like Tarcho. He thought that Thalius would have said it was a cosy piece of finitude sliced out of an infinite and troubling world.

He was woken in the dark, by a big hand gently shaking his shoulder. Unthinkingly he turned limp, imagining he was back in the mine. If you fought the dirty men they made it worse for you. But he was still in Banna, and it was Tarcho.

'Come on. Get dressed. I've something to show you.'

Outside the house the fort was a pool of shadows. The only sound was the coughing of a soldier on sentry duty somewhere on the fort walls. The sky was a deep blue-grey, a warning of the dawn, and the light reflected from dew on the cobbles.

Tarcho led Audax to a watchtower on the wall, and showed him a ladder. 'Take care,' Tarcho whispered. But Audax was used to ladders in the pitch dark, and climbed up more easily than Tarcho himself.

They arrived on the narrow platform at the top of the tower, alone. Up here the air was fresh, crisp with dew, and the customary piss-stink was dissipated by the green smell of growing grass.

Audax looked out over the countryside. The fort was on an escarpment, and looking south he could see how the land fell away to a deeply cut valley where a river gurgled. A steamy stink rose up; the fort's bath house had been built down there near the water.

And when he looked east and west, Audax at last saw the Wall itself, built into the outer shell of the fort, striding in great straight-line segments across the country. Where the gathering light in the east caught the curtain's southern face, the pale stone shone. Buildings and forts studded its length, and Audax could see hearth smoke rising, as if it was one gigantic house.

It was centuries old, Tarcho said proudly. 'My own great-great-great-something-grandfather worked on it. He was called Tullio. I know his name, you see, because it is written on stones set in the Wall itself. He came from Germany. And his sons and grandsons have served on the Wall ever since. Here is the Wall, all patched up, still serving its purpose eight, nine, ten generations later, after most of its builders' names have been forgotten. What men they must have been in those days, that their vision still shapes our age today! What heroes! And one of them was my grandfather.'