His strength recovering, and as Tarcho had still not come back with the boy, Thalius felt a little restless. He put down his cup and, under Volisios's uneasy stare, prowled around the room.

It was a working office heaped with paperwork. One pile of lawyers' letters was weighted down with a bit of quartz shot through with gold, a pretty stone brought up from the earth no doubt at the cost of much human suffering. There were few personal touches, but there was a small lararium, a household shrine, with tokens to gods Thalius didn't recognise. But in among this pagan clutter there was a rough Christian fish symbol, a brooch done in a bit of bronze wire. Such mixings-up of creeds were common. Despite the Emperor's promotion of the faith Christianity wasn't compulsory, paganism not a crime, and Rome's cheerful pantheism was, for now, able to absorb Jesus as just another god.

The most interesting item in the room was a framed set of coins. They had been struck during the reign of Carausias, and showed the usurper's proud profile alongside the legitimate continental emperors he had claimed as his 'brothers', and icons that portrayed him as a fulfilment of Virgilian prophecies of a saviour of Rome.

Volisios was still watching him uneasily.

'So,' Thalius said, 'you were a supporter of Carausias?'

'Never,' Volisios said quickly. 'I just collect the coins. They are already rare, and quite valuable. Just think! The coins of a British emperor. That's all this is, Thalius, a coin collection.'

Thalius had never been one for bear-baiting. 'Oh, you needn't worry, man. I'm no government spy. Though I've no doubt you've plenty of murky secrets-what, a coin hoard? A few sons secreted away to evade the labour levies?'

Volisios shrugged. 'Doesn't everybody try to get away with a little? The taxes these days are too much to bear. And it's all corrupt anyhow.'

'You're right about that,' Thalius said grudgingly. 'Too many are on the take. The Emperor is coming to Britain to raise troops for his war with Licinius.' Licinius ruled the eastern half of a sundered empire. 'But I'm hoping that while he is here he will do something to clean up our civic life.'

'And lower these wretched taxes,' Volisios said.

'Oh, I doubt even the Emperor will be able to manage that,' Thalius said dryly, and he turned back to the coin images of the doomed usurper.

Thalius remembered the rule of Carausias well; he had been in his twenties when Carausias took power. The man had commanded the British fleet, responsible for intercepting Saxon pirates as they sailed across the sea from north Germany. It turned out he was allowing the raiders through, then robbing them as they tried to return home. When the provincial government not unreasonably challenged this policy, Carausias led his troops in rebellion. For a decade he (and later his own usurper) held off the continental Tetrarchy and ruled as emperor in Britain.

It was all very exciting, and Carausias, charismatic and imaginative, had been popular. His island domain was a British Empire sustained by sea power, with a 'Rome' of its own at Londinium, and Thalius, rather thrilled, had wondered if this was a glimpse of the future.

But the hard truth was that for Thalius, at that time a young man assuming his own burdens as part of the curia of Camulodunum, a change of hierarchy at the top of society had made no practical difference. And the fragile rebellion had been decisively crushed when Constantius Chlorus led his massive Invasion of Britain, and reclaimed the island.

Ironically Constantius Chlorus was destined to be the father of another British-based usurper. Constantius had been one of the Tetrarchy, a college of four emperors who ruled jointly-a system optimistically designed to stabilise the imperial succession that was never likely to survive the abdication of its founder, Diocletian. On Constantius's death the British army elevated his son at Eburacum. After a complicated series of political, dynastic and military conflicts, the son had become master of the west, though he still faced his rival in the east. But, of course, as a winner he was no longer regarded as a usurper.

Few of Thalius's friends had studied history as he had, and few knew that poor, charismatic, doomed Carausias had been only the latest of a series of usurpers across the empire. The first British-based usurper had been a governor, African-born Clodius Albinus, who, seventy years after Hadrian, took the British garrison to the continent, only to be destroyed by the emperor Septimius Severus. Severus had split the province in two, so that no governor could ever be so powerful again. A century later Constantius Chlorus had split the provinces again; there were now no less than four Britains. But the years between Severus and Constantius had seen little peace.

Thalius had concluded you could trace all this instability at the top of the empire back to Hadrian and his Wall, completed nearly two centuries in the past.

The Wall itself had proved a durable limit to Rome's ambitions. Though Severus, conqueror of Clodius Albinus, had ventured beyond the Wall, reaching the furthest point of the highlands, his campaign dissipated on the bleak high ground-just as had Agricola's before him. The Romans had never again attempted to conquer the far north.

But there were consequences of the end of expansion. With no new provinces to plunder, the empire's only income was the taxes and levies it raised on its peoples. Meanwhile beyond the static frontiers, even in Caledonia, the barbarians had the chance to form new and more coherent federations, and were becoming an increasing threat.

So while the empire's acquisition of wealth had declined, the cost of defending it was rising-and taxes inevitably rose, generation on generation. The gentry fled their expensive responsibilities in the towns for grand estates in the country, while the poor were driven into evasion, criminality and destitution.

The army was changing too. Posted for ever at static frontiers, the troops understandably became more loyal to their local commanders than to any distant emperor-and generals who had once looked beyond the borders of the empire for glory were now forced to look inward to pursue their ambitions. These centrifugal tendencies spun off one usurper after another, even in Britain. It had taken a new breed of tough, ruthlessly competent soldier-emperors to pull the empire out of a crisis that might have been terminal. But it seemed to Thalius that the character of the empire had been transformed in these trials-and now, under a new emperor, it might be transformed again, even more drastically.

And Thalius, amateur historian, had looked further back in time still. Many of his acquaintances imagined that Constantius's Invasion of Britain two decades ago had been the first Roman assault on the island-as if Britain had always been Roman. But there had been a history, of a sort, even before Claudius's adventure three centuries ago. Thalius had read, fascinated, of British rebels who had sought to throw off the yoke of Rome, calling themselves Brigantian or Iceni or Catuvellaunian-names Thalius had thought only referred to Roman administrative units. He had no idea what the deeper history of these lost nations might have been. He had been astonished to find in his family research that he himself was, at least partly, of Brigantian blood.

Now the British saw themselves as Romans-and if they rebelled, like Carausias, they did so within the system rather than trying to overthrow it. The people of old, his own ancestors, had had minds of a different quality from the modern, Thalius thought. He wondered how much else had been changed, or lost, in the Roman centuries.

There was a noise outside. Volisios's office had small blue-tinted glazed windows; looking out, Thalius saw a band of workers being brought up from a mine shaft and marched off to some rough barracks. As they passed they glared at the overseer's hut. Thalius shivered, despite the heavy irons that bound the slaves' legs and necks.