On the second day Subh's son Ibrahim rode alongside Peter for a while. On his handsome charger he looked down on Peter, who thudded along on the back of his reluctant old mule. Ibrahim was provocative from the off. 'You are the only Christian in this caravan of Muslims. Even the muleteers are Muslim. Only you, out of place, and far from home. It is a certain kind of weakness, I believe, that drives a man to seek out the company of strangers. Why are you here, Christian Peter?'

'For the scholarship.'

Ibrahim hawked and spat. 'You could have enjoyed your scholarship without leaving London. Do you have a wife in London? A woman you love?'

'No wife or lover.'

'A boy-'

'I have no interest in boys, Ibrahim.'

'Then what are you fleeing from?'

'I'm fleeing nothing. I'm travelling in hope. I am following a loose thread in a tapestry, letting it lead me where it may. Your mother understands, I think.' Peter grew impatient with his pressing. 'Why should Christian and Muslim not share the adventure of life together? In Toledo, Christian and Muslim scholars meet and work together every day.'

'Under the banners of a Christian king.'

'Perhaps. But in the days of the caliphate Christian scholars similarly flocked to Muslim Cordoba.'

Ibrahim said, 'But there was no assimilation. Five centuries ago the Moorish armies marched north. The whole of Spain became a Muslim country, and Christians lived in a Moorish land. Now the Christians are scouring their way back down from the north, and Muslims will have to survive in a Christian country, as my family survived in Cordoba. No matter how long they cohabit, Muslim and Christian will not mix, any more than water and oil, whether there is more of the oil or more of the water.'

Peter considered arguing against this. But the evidence for it stood all around him, the bristling relics of warfare sticking out of the ground like broken teeth. 'All right. But here we are, Ibrahim, riding side by side. We don't have to fight, do we?'

Ibrahim eyed him, his eyes as bright as the sky. 'Perhaps not. But keep your gaze fixed to the backside of that mule in front of you, and off my mother's.' And he galloped away to rejoin his friends.

In the morning Thomas woke early, barely able to believe that he was really here. But even before he left his bed the wail of the muezzin calls told him that this holiest of cities was not Christian, not entirely.

Saladin came to find him. 'We should take that walk while the day is young. I don't imagine you will enjoy the noon heat.'

So Thomas ate his breakfast and hurried through his prayers, and followed Saladin out into the city.

Jerusalem was extraordinary, overwhelming, baffling, a warren of streets, a stew of history. On the Temple Mount, the gold cap of the Dome of the Rock gleamed like the sun brought down to earth, and beside it the silvery al-Aqsa mosque was light, airy, a dream in stone. The Muslims called the Mount al-Haram ash-Sharif: the Noble Holy Place. The Muslims were relative newcomers to Jerusalem, compared to the Jews and the Christians, but even they had already been here centuries.

The city was full of churches, of course. Some of them, built by the crusader kings of Jerusalem, were quite modern, with ribbed vaults and pointed arches, and would have graced any city of western Christendom. Others were older, more squat and monumental. These were Roman, many of them built during the long centuries after the legions had left Britain. Thomas poked around these buildings, fascinated.

But as he talked and analysed and speculated, Saladin hardly spoke. To him, Thomas thought, history meant little. Jerusalem was nothing but an arena for the warfare he expected to dominate his life, as it had the lives of his ancestors since Robert, who had come here with the First Crusade that had swept through the Holy Land like a fiery wind.

The Christian states the crusaders planted here had survived three generations, until fortune threw up a great Muslim commander: a former vizier from Egypt, al-Malik al-Nasir Salah ed-Din Yusuf, known to the Christians as Saladin, who marched on Jerusalem. Even Richard the Lionheart was not strong enough to take back the holy city. Today Jerusalem was ruled under an agreement negotiated by the German emperor Frederick II, the nominal king of Jerusalem. The city had been left unfortified, and Muslims were allowed to remain; it was a shabby deal. Still, some of the old Christian families, like Joan's, had crept back into the city which they had regarded as their home for generations.

So much for warfare. But, Thomas asked himself, what kind of people were these folk of the Outremer?

The boy Saladin was an ardent Christian, that was certain; here on the front line of Christendom you would expect a sharpening of the faith. But he looked more Saracen than English, with his flowing robes, his dark skin, and Thomas had heard him jabber Arabic phrases as easily as he spoke his stilted English. Thomas guessed that there had been a few infusions of Saracen blood into his line over the generations. Saladin's deepest ancestry was English, but transplanted to the soil of Palestine for generations his kind had become something different, neither English nor Palestinian, something new in the world since the time of Robert.

And these new people of the Outremer were isolated, in a way that the Norman invaders of England, say, were not. King William's sons had taken English wives; after nearly two centuries their assimilation was complete. But Normans and English were both Christians. In the Outremer the remnants of the crusader kingdoms were islands of Christianity in a sea of Islam. This was Saladin's home, but his family would always be out of place here. And Thomas sensed that Saladin knew it in some deep part of his soul.

The day grew warmer, and the old city became a stone bowl full of hot, dry air. Thomas was grateful when Saladin took pity on him and led him back to the relative shade of Joan's house.

As they rode steadily south-west and closed on Seville, the caravan entered Muslim lands.

Before the city they came upon an extensive Moorish army camp. It was a town of tents, men, horses, mules and camels, planted near the river bank; flags bearing the crescent hung limply in the heavy air. Weapons were piled up in huge mounds, shields and crossbows and spears supplied by the factories of Seville. Peter could hear the pounding of war drums, not coordinated but still a chilling sound, like distant thunder.

A party of troops rode out to meet the caravan, an officer and a small escort of hard-eyed desert horsemen. The officer wore a coat of quilted felt over mail armour, while the horsemen wore white robes and turbans; they carried spears and shields shaped like hearts. Subh had a letter, a conduct of safe-keeping from the emir in Seville. After an exchange of gifts – a bag of gold from Subh, some water carriers from the soldiers – the troops rode off back to their camp.

The muleteers made a wide detour away from the river to avoid the soldiers, and Peter was relieved, for everybody knew that the best-controlled soldiers were always liable to a little plunder, robbery and rapine. But he studied the camp, fascinated at the sight of a genuine Moorish army. It was odd that there were no wheeled vehicles to be seen, but along with mules the horses were gathered in great herds – imported from the Balearic islands, said Ibrahim, only the best for the army. And out of the ranks of the horses and mules rose the necks of haughty camels, brought over from Africa.

The core of the army was made up of levies raised on the provinces of al-Andalus, or what was left of it – cavalry from Granada, for instance. Ibrahim pointed to groups of soldiers with dark faces, 'the silent soldiers' he called them dismissively, many of them Berbers who spoke no Arabic. But the most ardent fighters of all, said Ibrahim, were the volunteers who came here from across the Islamic world to 'the land of the jihad', as many Muslims called Spain. It was just as the crusading armies were made up of volunteers from across Christendom. Peter was awed to imagine the energies of two continent-spanning civilisations focused here on this place, this point in space and time.