He said impulsively, 'Agnes – never mind prophecies. I still don't understand. What made you do this? Why run away – why throw away your life – why wall yourself up in a cell?'

'For the love of God.'

There must be more. 'And?'

She sighed. 'And because I thought I would be safe,' she said softly. 'If I am in here, far from Oxford, encased in stone, he could not reach me again.'

He thought he understood at last. 'Our father.'

'Yes. It was not until Geoffrey came that I learned he was dead.' She closed her eyes.

'What did he do, Agnes?'

'He was maddened. He was drunk. He didn't know what he was doing. I forgive him; I have prayed for him. But I was ten years old. I feared that if I stayed, if I became a woman, and if his seed was planted in me – I left to save myself, and him, from that terrible sin.'

'Oh, Agnes. I didn't know. You say I protected you. But I failed, I failed-'

'It wasn't your fault, but his. Agnes is the name he gave me. But Agnes was a holy virgin. I am no Agnes.'

Impulsively he pushed his hand through the slit window. Tentatively his sister clasped his fingers, and then he felt the softness of her cheek on his hand.

Later he asked Geoffrey about the trench in the cell. It was Agnes's own grave, Geoffrey said, a grave she scraped every day, for an anchoress was commanded to keep her death before her eyes at all times. Agnes would live and die in her stone box, and when her life was done she would lower herself into her self-dug tomb.

Grace Bigod and Friar James had come to Seville to meet a man called Diego Ferron, a Dominican friar with contacts in the court of the Spanish monarchs. He was attached to a monastery outside the walls of the city, and had offices, it was said, in the palace complex of the Alcazar itself.

Ferron kept them waiting for days after their arrival. The date he suggested for their meeting, he told them in his note, was 'suitable for our joint purpose'. James didn't know what this meant.

On their tenth day in Seville, Grace and James were at last summoned to Ferron's presence, at a private house in an old part of the city. When they arrived early at his house, they were led by a barefoot servant through a complicated archway into a garden, where water bubbled languidly from a fountain into a pool full of carp. The house was clearly Moorish, presumably abandoned by its owner on the fall of the city more than two hundred years ago. At least the Christian owners of this place had taken care to preserve what they had taken, though the furniture, lumpy wooden chairs, benches and low tables, would not have looked out of place in the home of a well-to-do Englishman, and walls which still bore Arabic inscriptions in praise of Allah were now studded with crucifixes and statues of the Virgin.

Friar Diego Ferron walked in briskly, introduced himself, ensured they had been served with tea and sweetmeats, and sat upright on a severe wooden chair. His habit was adorned with a magnificent black and white cowl made of some very fine wool. He was perhaps forty, his tonsured hair jet black and well groomed. He was a handsome man, his features sharp, his eyes brown, and his skin, shining with oils, was so dark that if not for his vestments James might have thought he was a Moor himself.

James was uneasy in his presence. When the brothers in Buxton had learned he was to meet Dominican friars in Spain, they had laughed. 'They're an odd lot, those Dominicans,' one comfortable old friar had said. Saint Dominic, fired by his experience of the Albigensian heresy in France, had dedicated his order solely to the task of fighting heresy in all its forms. 'And they're worst of all in Spain. Mad as a bat.'

Ferron did not strike James as mad, but businesslike. Certainly he did not waste any time on pleasantries.

He focused his attentions on Grace. 'First let me be sure you understand my role in the court of our glorious monarchs, Fernando of Aragon and Isabel of Castile.' He spoke fluent Latin. 'You wrote to court to request an audience with Tomas de Torquemada. The friar is a Dominican colleague of mine, and was confessor of the Queen.'

'Yes-'

'Friar Torquemada is now working with the Inquisition. The Queen's confessor is now Friar Hernando de Talavera, a Hieronymite. Pious, ascetic – a good man. The second of the Queen's chief prelates is Cardinal de Mendoza, the archbishop of Seville. These persons will be involved in assessing your proposal for the court. I myself am on the staff of Friar Torquemada.'

'You work for the Inquisition, then,' Grace said.

'Yes. But I have good relations with both Friar de Talavera and the archbishop, as well as Friar Torquemada, and so he passed on your request to me as a suitable first point of contact.'

This politicking among holy men baffled James, and dismayed him obscurely.

Grace bowed. 'I'm sure we will be able to do business together, brother.'

'That's what I'm here to find out,' Ferron said slickly, quite coldly. 'For it is business we are talking about, isn't it? You are here to sell arms to the monarchs. These weapons, the Engines of God as you refer to them.'

'There is more to it than that-'

'We have arms. We have cannon, we have arquebuses.'

'But nothing like the weapons I can offer you,' Grace said urgently. 'The engines are founded on the words of a prophecy, retrieved by my ancestor Joan of the Outremer from a cache beneath the mosque of this very city. The designs have been developed in secret for two hundred years by Franciscans, followers of the sage Roger Bacon – perhaps you have heard of him. Brother James here has studied the developments closely and can tell you all you wish to know.'

Ferron's glance flickered over James. 'I already know the most salient fact. That these weapons of yours are decidedly expensive.'

'Decidedly better than anything you have. And decidedly what you need for the coming war. I do not mean the conflict with the Moors of Granada. I mean the war to end wars that will follow.' She paused, her face intense, beautiful. 'Brother, I know of this, deep in my bones. My family is of the Outremer, the Holy Land – we lived in Jerusalem itself. We were expelled by the Saracens in the same decade as Seville fell to the Christian armies. This was over two hundred years ago, and we still bear the scars in our souls. They are scars of the long war with the Muslims which has been waged since the death of Muhammad himself. And it is a war Christendom is losing.

Ferron sat back, startled.

She had seized control of the exchange, James saw. They were the same age, roughly, Grace and Ferron. Both strong, both determined, both combative. They would be formidable enemies, still more formidable if they became allies.

James knew, though, that any cold-eyed observer of history would draw the same conclusion as Grace. Ever since the loss of Jerusalem in Joan's day, Christendom had been on the retreat.

The problem was the rise of the Turks. A decade after Seville fell to the Christians, the Mamluk Turks defeated the Mongols – the first significant defeat suffered by the nomads across three continents. It was a turning point for the Islamic empires. The Mamluks, rampant, marched on; within decades they had obliterated the last trace of the old crusader states. Eventually the Mamluks fell to new waves of Mongol invaders. But out of their shattered polities rose a new nation of Turks called the Ottomans, who dismembered what was left of the old East Roman domains. The last Roman emperor died fighting for Constantinople, when the old city fell in 1453. A jubilant Sultan Mehmet crowed that Rome itself was next, that soon he would be feeding his horses on oats from the high altar of Saint Peter's. And in the year 1480, just a year ago, as if making good that promise, Mehmet had assaulted Italy.