'The testudo,' Ferron said weakly, 'is astounding. Devilish!'

'Not the devil,' Grace said smoothly. 'It is all the work of man, his imagination divinely inspired.'

'But how is it possible for such a weight even to drag itself over the earth? There must be a herd of horses in there.' He cupped his ears in gloved hands. 'And the noise-'

'Not horses. Bacon's black powder.' And, sitting beside Ferron in the wooden viewing stand, she tried to explain how the gunpowder had been harnessed into an engine. 'There are a series of pistons. When the gunpowder charge explodes above each piston, air is forced out of a chamber of iron, and the piston is dragged up, as a man inhaling may draw a feather into his mouth. That motion is translated into a turning of the great wheels, by a complicated mechanism James could no doubt describe for you. And so the steel beast travels forward, powered by a beating heart, each pulse a detonation that could kill ten men…'

While Bartolomeo Colon stared, fascinated, it seemed to be too much for Ferron. He held his hands over his ears, flinching from each new explosion. 'Devilish,' he repeated. 'Devilish.'

She tried to distract him with the manufactory's new sort of arquebus; one of them was set up on display before them. 'Then consider this, brother. The old sort of hand gun, as Isabel is deploying against the Moors even now, is slow to reload, and unreliable to fire, for you must apply a flame to the powder that propels the shot. Now we have a new sort of gun – based, again, on the designs in the Codex – which is fired not by flame but by a spark.' She showed him how, when a trigger was pulled, a hammer slammed a bit of flint against a steel plate; the resulting sparks were funnelled into a chamber to ignite the gunpowder.

Ferron was distracted by the glistening mechanism as she operated it. 'I see,' he said. 'I see.'

'It is still difficult to reload – we will work on that – but the reliability is so much improved, the weapon is so much safer, that it will be as if we have double the number of soldiers in the field. And furthermore-'

'What,' Ferron said, pointing, 'is that?'

It was a woman – young, scrawny, dirty. Grace had no idea who she was. She was running. She fled towards the battlefield. Grace could not have imagined a more unexpected sight.

And now monks followed her, grimy, blinking in the light. They too ran towards the noise and smoke of the field, not pursuing the girl, just running. But one of them called over his shoulder to the spectators in the viewing stand. 'The manufactory! Get away, my lady – the manufactory!'

'Dear God,' Ferron said.

Grace was bewildered, unable to understand what was happening. 'I think-'

The explosion was a roar, all around her. She was thrown forward onto the ground, helpless as a doll.

From the air, James saw fire erupt from the ground, a line of searing fountains. Monks and novices squirmed out of hatches like moles emerging from their holes, and ran off. James understood immediately. The fire was breaking out of the ground through the air vents of the underground manufactory. The explosions must have come from within the compound. It was the store of gunpowder, it could only be that. Some accidental spark had ignited it – or perhaps, he thought suddenly, it had been deliberate.

James had to concentrate on his own flight. His mechanical bird was dipping towards the ground. He had only a few heartbeats left in which he could control his descent. He scanned the ground anxiously, looking for a clear space to land.

But a fresh set of explosions broke out over the location of the main manufactory, distracting him, and James saw bones hurled into the sky. The gunpowder must have broken open a plague pit. It was an extraordinary, unnatural sight to see those bones go flying up into the air and then fall back, a grotesque parody of the Day of Judgement.

Harry, with Abdul and Geoffrey, had been watching the display from a distance, with appalled fascination. The wooden bird in the sky especially was an awful, unnatural sight.

But when the fires began to erupt from the ground they all knew something had gone badly wrong. Abandoning all attempts to conceal themselves, they ran towards the party coming from the viewing pavilion.

They met the others not far from the entrance to the manufactory. The explosions had stopped now, but smoke still poured from the ground. No more monks clambered out of the hatches, and Harry wondered how many had died that day.

Diego Ferron was unmistakable, a tall, pale cleric. He was holding a woman by her hair, a wretched, skinny girl in a grubby white gown. Beside Ferron and his captive was Grace Bigod. She was a hard woman of nearly fifty, her face smeared with soot and twisted in fury. It was the first time Harry had met this remote cousin.

Ferron seemed surprised to see Abdul with two strangers, but his rage overwhelmed him. In accented Latin he cried, 'Ruined! Destroyed! Centuries of work lost!'

'Not lost,' said Grace, her voice trembling, 'Just delayed. We have lost our engines, but those in the field survive, and we have the designs-'

'Lost because of this Christian witch!' He twisted the girl's hair and threw her to the ground.

She lifted her head. She looked straight at Harry. Her hair fell away from a bruised face.

'Agnes!' He could not have been more shocked if his sister had been raised from the dead. 'But you are in your cell in York.'

'Evidently not,' she said. Her voice was a scratch, and she coughed, her lungs full of smoke.

Grace looked at Harry and Geoffrey. 'Who are you?'

Harry ignored her and spoke to his sister. 'And you – you caused this destruction?'

She whispered, 'You are a good man, Harry, a good brother. But you are not strong enough to do what is necessary. I prayed. God spoke to me. My mission was clear. It was worth breaking out of my cell for this, wasn't it?' She forced a smile, and suddenly she looked as she had when she was a little girl.

His heart broke. He stepped forward. 'Oh, Agnes-'

But Ferron blocked his way. 'Keep away. This witch is for the Inquisition. Keep away, I say!' And he brought his gloved hand slamming down on the top of Harry's head.

The world peeled away into darkness.

Seville was cold that February morning, and the wind that funnelled along the Guadalquivir was biting. It was a disappointment for Geoffrey, who had at least expected to be able to warm his English blood as a reward for undertaking this hellish trip.

It was a relief to get out of the open air and duck into the great cathedral, where he was supposed to meet Abdul.

In the still, incense-laden calm, he genuflected and crossed himself. The cathedral was a cavern of sandstone and marble. His gaze was drawn upwards to a vaulting roof that was filled with a golden light cast from huge stained-glass windows, a hint of heaven. There was nothing on this scale in England. The cathedral was a sink of wealth; it was expensive, tacky, uplifting, crushing; and it was certainly a monument to the untrammelled power of the Church in Spain.

Abdul Ibn Ibrahim met him just inside the doorway. His turban and long Moorish cloak looked thoroughly out of place in this Christian space.

Geoffrey greeted him. 'I'm surprised they let you in.'

The Moor shrugged. 'We Muslims are not barred. Perhaps the priests hope that I will be converted by the sheer stony mass of this place.' He grinned, comfortable in himself. 'So you arrived safely. What do you think of Spain, of Seville?'