Damon picked his way along the rubbish-strewn road quite cheerfully. All this was normal to him. To Isolde and Nennius, though, the whole place stank like the vast garbage heap it was. And there were so few people that Isolde felt like a child creeping through a huge house abandoned by adults.

The bishop's palace was one of the grander buildings. It was a townhouse about thirty years old, built in something like a classical style, with separate blocks set out around an atrium. Isolde learned it had once been owned by a diocesan tax official.

Bishop Ambrosius himself was here to greet them. With receding silver hair, richly dressed in a ground-length purple robe, he was about Nennius's age, but he looked as if he could still throw a healthy punch. When he took her hands Isolde felt reassured for the first time since she and Nennius had left Rome.

After days on the road they were all hot, dirty, hungry, and the stench of the city lingered even here. There was no bath house, but the house had underfloor heating and hot water, and after an hour of pampering herself with her scents and creams Isolde felt almost human. She joined her host and his guests in a large, well-appointed triclinium.

The meal they were served relied heavily on meat, mostly mutton and pork, but there were olive oils and dates and a rather good fish oil, imports from the continent which, the bishop said, were a luxury these days. No wine, though, and Ambrosius apologised for serving them watery British beer, but it was strong enough to warm Isolde's blood.

Her father, as usual, seemed to have forgotten she was here, and made no effort to include her in the conversation, and nor did Damon. But the bishop was gracious to her, and his servant, another young monk, was attentive to her needs. In the light of the low fire, as the aches of the journey were soothed away, Isolde was content to let the talk wash over her.

Ambrosius and Nennius were churchmen of a similar age, both in their late fifties, one from the heart of the empire, the other from its now-amputated limb, and they had lived through tumultuous times. And, like all men of their age, Isolde thought fondly, they believed the world was in decline, from a Golden Age a few decades before. Born a generation after Constantine, now known as 'the Great', they spoke reverently of the first Christian emperor, whose reign had been an interval of comparative peace.

But all over the known world the weather was bad. Even within the empire the great crops of wheat and millet which fed the large urban populations began to fail. Among the barbarians, as famine descended, pulses of refugees washed out of the heart of Asia, driving others before them, to press ever more relentlessly on the borders of the empire.

Constantine himself had stabilised the border by allowing barbarian peoples to establish new homelands inside the empire's borders-vast numbers of them, for instance no less than three hundred thousand Sarmatians. This policy of 'federation' brought peace for a while, but there was much muttering about whether the empire could absorb such huge influxes.

There was trouble with the barbarians in Britain too. When Ambrosius was a boy of five, there was a 'Barbarian Conspiracy', a major invasion from several directions at once: by the Picts across the Wall, the Scots and Irish and Saxons from across the Ocean. For a whole year order was lost. Ambrosius and his family, he said, huddled inside the town walls as the children told each other blood-curdling tales of the baby-eating foreign savages who roamed the countryside.

Meanwhile after Constantine's death fratricidal war between his three competing sons brought renewed turmoil at the heart of the empire. Britain threw up usurpers, such as one Magnentius, who had killed one of the sons, only to be defeated by another, at Mursa in Pannonia. The empire once more became an arena of conflict between strong men and their armies. Then, when Constantine's treaties were repudiated by one barbarian group, the Visigoths, decades of simmering conflict finally led to battle at Adrianopolis-a battle the Romans lost, at a terrible cost.

Times had changed, and a weakened empire could no longer afford such bloodletting, and after Mursa and Adrianopolis the Roman army had never been able to recover its strength. The emperors employed barbarian federates in their armies, and still more alien peoples settled within the borders-following Constantine's precedent, but now the process was all but uncontrolled. Thus the army was barbarised, and the empire was hollowed out by foreign polities.

The British felt increasingly exposed, and its elite and officer corps threw up one usurper after another-roughly one a generation, as young rebels dreamed they could do better than their fathers. And as soon as he took power each of them headed overseas, taking yet more troops with him from the British garrison. One, grandly named Magnus Maximus-'the Great, the Greatest'-killed another emperor, and had the dubious honour of being the first western ruler to order the execution of a Christian heretic.

But the troops stripped from the British garrison by each failed rebellion, and indeed by each officially ordered transfer to reinforce the centre, were never returned. Without the army to enforce collection the taxation system began to break down. People hoarded their coins in the hope of better times, and a barter economy sprang up. But as demand dwindled there was much destitution, and Ambrosius, by now a young priest, found himself ministering to the starving poor.

Then, one New Year's Eve, a vast horde of Alans, Suebi and Vandals crossed the frozen Rhine, and pushed their way into Gaul and Iberia.

Britain was cut off. Now no money was coming from the central treasuries to pay the remaining British troops. The imperial standard still flew over the forts and towns, but there was little left of the Roman army beneath it. After a succession of bloody, panicky coups a low-born general who styled himself 'Constantine III' put together yet another ragged army, and just like his predecessors immediately crossed to the continent. Constantine was the last throw of the dice, the last British attempt to save Britannia.

As Constantine's campaign on foreign fields descended towards defeat, another turning point was reached. This time it wasn't the officer corps or the ruling elite in the cities who rose up but the comparatively poor and lowly. What was the point of being taxed white by a centre which was better at producing usurpers than keeping out the barbarians? The revolt, once it broke out, spread like wildfire. The tax collectors were expelled from their offices and plush townhouses, and then the Christian poor turned on the still-pagan rich. The diocesan government collapsed. The middle-ranking officials in the towns and the four provincial governments decided to join the rebels, and legitimised and organised the revolt.

This 'British Revolution' was less than ten years old, and Isolde thought she detected pride about it in the calm voice of this churchman. Not that things were perfect. The four provincial governments had started to develop independent armies, mostly made up of Germanic mercenaries. But civilisation had been saved until the day came, as it must, when Britain was reunited once more with the centre of the Roman world of which it had always been part.

'And it was a blow for freedom,' Nennius said, enthused, eager for revolution in a way, Isolde thought a little sourly, only a comfortably plump old man far removed from the action could be. 'Isn't that a deep tradition in Britain? Why, you could say it informs the work of Pelagius himself.'

'Ah, but never let Augustine hear you say that…'

Isolde had met Pelagius herself once, as a little girl. About ten years older than her father, born in Britain, he was not a cleric, but he had developed forceful views about the direction of the Roman church, which he saw as corrupted, immoral and slothful. And he took great exception to the teachings of Augustine, a bishop from Africa, who argued that human beings were born fundamentally flawed, and that human actions depended on the will of God. Pelagius insisted that humans were essentially good, and were responsible for their own moral advancement.