They walked still deeper into the mosque. In places there were multiple arches, arches built on top of others like children standing on each others' shoulders, all exquisitely carved. And the Mihrab, another arch adorned with gold leaf, was like a gateway to paradise. Its materials were a gift to al-Andalus from Constantinople, said Moraima.

Lost in the mosque's cool spaces, Robert realised he hadn't been aware of the two boys, Ghalib and Hisham, for some time.

'Oh, they got bored long ago,' said Moraima when he mentioned them. 'Come. Let's get some air.'

When Sihtric was done with the vizier, he had suggested to Orm that the two of them should take a ride, further out into the country.

Orm mounted his horse suspiciously. 'Where are we going?'

'You'll see. Go ahead, boy… So, what of Robert? He seems drawn by the Moorish world.'

'He's his mother's son, may God help him. He's a confused young man – more confused than he knows. But it's the fact that he's drawn to your daughter that concerns me more. No good will come of it,' muttered Orm.

'He's his father's son too. You were just as young and foolish once, Orm.'

'Yes,' Orm snapped. 'And it led to tragedy.'

Sihtric said testily, 'But if we ban them from seeing each other they will just ignore us. We'll have to find a way of coping with things as they unfold.'

'So what do we do in the meantime?'

'I suggest we pursue the business for which you came all this way.' He grinned. 'I think you are going to enjoy this.'

They topped a small rise, and Sihtric reined in his horse. He pointed. 'There. What do you see, among those olive trees?'

Orm stared. There was much activity going on in the olive grove. The centre of it seemed to be a kind of machine that nestled among the trees, a long cart that rested on three sets of widely spaced wheels. A large wooden crescent-shape dominated one end, and its upper surface was meshed by ropes and gleaming metal. The whole was obscured by a kind of scaffolding, through which a boy clambered, fixing ropes.

The machine was the product of a kind of open-air workshop, Orm saw now. Men and boys moved between furnaces, lathes, piles of timber, and tables heaped with gleaming metal components, and scholars came and went between rows of tents among the olive trees.

'Quite a sight,' he said, non-committal.

'It is, isn't it? What are we building, do you think?'

Orm shrugged. 'Some kind of wagon?'

'Come, Orm, stretch your limited imagination. Just look at it. Never mind the scale: tell me what you see.'

The shaft, the bow, the ropes. 'It looks like an arbalest,' Orm said. 'Which the English call a crossbow…' But an arbalest was a gadget small enough for a man to hold in his arms. This machine sprawled across a field, and had a boy actually walking along its back. Orm muttered prayers to the pagan gods of his childhood. 'By all that's holy-'

'Oh, there's nothing holy about it.'

'Aethelmaer?'

'Aethelmaer. Come, let's ride down.'

Orm remembered Aethelmaer.

In the last days of the reign of King Edward the Confessor, Sihtric had attached himself to the court of Harold, Earl of Wessex, as a priest-confessor – and as a prophet of sorts. He believed he was in the possession of a prophecy already four centuries old, a calendar-like vision called the Menologium of Isolde, whose sole purpose was to ensure an English victory over the Normans in the year of the great comet – the year of Our Lord 1066. Not that it had done much good. Harold, who had refused to take all the prophecy's advice, had fallen to defeat by the Normans.

But during his career as a court Sibyl, Sihtric had learned of the existence of a rival.

'Aethelmaer! A fat, crippled monk from Wiltshire,' he said with some bitterness. 'Who had also been uttering prophecies about the comet. I've since found his very words, among his papers.' He quoted from memory: "'You've come, have you, O comet? You've come, you source of tears to many mothers. It is long since I saw you; but as I see you now you are much more terrible, for I see you brandishing the downfall of my country…"'

'And you summoned him to Westminster.'

'Yes. You were there, Orm, you remember.'

His useless legs stinking of rot and unguent, the monk had wheezed his way through an account of his prophecy – which turned out not to have been his at all, but gabbled out by a young man called Aethelred, who had been abandoned as a child, taken in by the monastery at Malmesbury, and then had his short, unhappy life curtailed by debauched brothers.

'But not before he had left behind a remarkable body of work, studied and preserved by Aethelmaer and others.'

'I saw them. Sketches of machines. Siege engines, catapults…'

'I call the designs the Codex of Aethelmaer.' Sihtric smiled. 'The Engines of God.'

Orm struggled to remember the fantastic designs he had glimpsed just once, decades ago, and had never understood even then. 'But they were just scribbles on parchment. In a lifetime of study, Aethelmaer could build none of them.'

'Not quite,' Sihtric said. 'He did try to build one, remember? That was how he became crippled.'

Orm shook his head. 'I never understood that. Why would you want to fly like a bird? Of course none of this means a thing unless you can actually build these mechanical marvels of yours.'

'True enough,' Sihtric said. 'And I think you would be pleased to learn that I too have failed like Aethelmaer, wouldn't you, Orm the Viking? Well, you're about to be disappointed.'

Orm stared at him. 'You mean the arbalest? Sihtric, can you really be developing gadgets, weapons, from the plans you stole from that mad monk?'

'Interesting choice of words,' Sihtric said. 'Stole? I hardly think so. You met Aethelmaer. Old, crippled, he could do no more than have his arse wiped by some young novice, and probably enjoyed it too.'

'Your talk is sometimes filthy for a priest,' Orm said.

'Well, I'm a filthy sort of priest. Anyhow Aethelmaer's laborious mechanical sketching would have gone no further when he died, if not for my "stealing". Am I not honouring his legacy, by trying to pursue the designs he left?

'And, "gadgets"? You make them sound like toys. These are engines, Orm. Engines of war – and, perhaps, of peace. Come now. Let me show you.' Sihtric spurred his horse forward.

Orm, overwhelmed, followed.

Robert and Moraima walked out of the mosque into dazzling daylight.

They headed down to the river, where waterwheels turned with a creak of wooden gears – Moraima said the wheels were called norias – and boats with colourful sails steered through the arches of the Roman bridge. On the bank, amid a clinking of coins, vendors sold food and water and parasols.

Moraima said, 'You were affected by the mosque, weren't you? Not everybody is. I think you're deep, Robert son of Orm.'

'Am I?' He laughed. 'Well, maybe compared to Ghalib and Hisham.'

'Now you're being jealous, and that's not deep. I can't always tell what you're thinking, though. What you're feeling.'

He thought it over. 'My time in Spain – I didn't know what to expect. That journey down through the country, the emptiness, the heat…' He was shy about this, but he tried to express himself. 'And when I walk into these marvellous places, the mosque, the palace – something inside me – it's like a bird fluttering in my chest.'

She astonished him by placing her hand over his. 'My father said you would be like this. You have your father's muscles, but the soul of your mother.'

'Whose soul does he say you have?'

'His sister's. My aunt, Godgifu, who died before either of us was born. And who loved your father, Orm.'