That was a shock. 'I knew nothing of that.'

She looked at him directly. 'Do you think love can cross generations?'

Confused, he turned away. 'I didn't come here for love. I came here because of my father's business with yours.'

'Yes. Our fathers are both veterans of Hastings, and I suppose something like that shapes you for ever. But the past is dead, gone, and they are old men. Who cares about our fathers' business? We are young. We are the future.'

He looked at her. 'You're talking about us.'

'What about us?'

He sighed, faintly irritated. 'There you go again. You drop hints, and when I respond you turn away and go all coy.'

She smiled. 'Don't tell me you don't like it. Would you like there to be an us?'

He gazed at her, hot in his tunic of English wool. 'You know I would, or you wouldn't talk like this.'

She said, 'But…'

'But we're so different. Muslim and Christian!'

'There are ways around that. The People of the Book are tolerated here.'

He grunted. 'Not in England, they're not. And you're becoming a scholar, as far as I can see. While I will never be anything but a soldier.'

'There's plenty of work for soldiers in Spain,' she said.

He smiled. 'Let's keep it simple. Do you think it would be a sin before God or Allah if I kissed you?'

'We could always find out.' She stepped towards him. Her skin was the smoothest surface he had ever seen, utterly flawless, and as her full lips parted he could smell the subtlest spice, a pepper perhaps.

But there was a rude cry. 'Hey, Christian! Take a look!' It was Ghalib.

Orm paced out the mighty weapon.

The body of the shaft was forty paces long, perhaps two wide, and mounted on three axles. The bow itself, twenty paces from tip to tip, was made of wood layers, finely cut and polished, that ran in smooth, pleasing curves, gleaming in the intense sunlight. It was like a section of a boat, perhaps, or a monstrous piece of furniture.

'You've used arbalests,' Sihtric called. 'Tell me about them.'

'The bow is usually made of metal.'

'Not here. We couldn't cast such an immense bow, and nor could it be bent back if we did. Look here, we use laminated wood, layers pasted and nailed together. We hired boat-builders from your Viking homelands.'

'The vizier's pockets must be deep.'

'And how do you load a crossbow?'

Orm grunted. 'Depends. The old-fashioned sort, you bend over, put your foot on a stirrup, catch the bowstring in a hook on your belt, and straighten up until you've got the string in the lock. The newer sort you have a little hand-crank to draw back the string. You put your bolt in the groove, and press a lever to release it.'

'The principles are just the same here. Look at this.' Long metal screws had been built into the body of the stock. 'These are used to draw back the string. It isn't hard; a single man can turn that wheel, down there. Or you can use a mule. And look, see how the carriage wheels tip outward? That's to give the base more stability. Here's a tilting platform so you can raise the bow, and aim the flight of the bolt. And here, you see, anchors lock the crossbow to the ground and reduce recoil.'

'And have you fired such a thing?'

'Only in tests. We're still refining the design. How well do your hand bows perform?'

Orm shrugged. 'A range of two or three hundred paces. You can pierce chain mail.'

Sihtric grinned. 'This beauty should have a range of miles. And it will pierce masonry. Thus, one of Aethelmaer's designs, all but realised – all but ready to be deployed in war. Tell me you're not impressed.'

Orm pursed his lips, and walked around the machine once more. 'Yes. I'm impressed by what you've built. But imagine this in war. It would take a long time to load, longer to haul it around the countryside to aim it – and it could be destroyed by a single burning arrow.'

Sihtric sighed. 'All right. But what if I told you that instead of just knocking down a bit of wall, my arbalest could deliver a bolt capable of laying waste an entire fort, even a city? A single bolt! What then?'

Orm grimaced. 'That sounds a fever dream, and an ugly one.'

'But Aethelmaer had such dreams, or Aethelred did. I can show them to you, sketched in the notes. Dreams of a super-weapon – Aethelmaer called it Incendium Dei, the Conflagration of God. Perhaps it is something like Greek fire – I don't know. But the only clues he left for it are encrypted, and it remains beyond the capabilities even of the scholars of al-Andalus to decipher. Later I'll show you the Codex itself – much study remains to be done on it. But first, come. I'll show you how we work here.'

Leading their horses, they walked away from the mighty arbalest and through the open-air workshop. Orm glanced with interest at the tools of the carpenters and metal-workers. He had ordered several swords in the course of a long fighting life, and had come to appreciate the metal-workers' art; casting the immense screws of the arbalest must have set them significant challenges.

On some of the tables models were set out, intricate wooden toys that looked like birds or beetles or fish. They were models made from designs even more astonishing than the great arbalest, Orm saw, engines that flew and swam and crawled. Some of them were sliced open so you could see the wooden skeleton within, and the bodies of tiny men working oars or hauling on wheels. The boy trapped in Orm's battered warrior's body longed to hold these gadgets, to play with them.

In one of the tents a wooden floor had been set down, a few paces across. Its surface was incised into rows, along which stones the size of fists painted black or white were lined up. Two scholars argued in rapid Arabic over a parchment. In response to their commands a boy jumped about over the board, moving stones from one row to the next. Occasionally he apparently made a mistake, and his reward was a volley of abuse, but when he got it right the scholars forgot the boy and argued over the patterns he conjured.

'So,' Sihtric said. 'Any idea what this is, Orm?'

Orm shrugged. 'Some kind of game?'

Sihtric snorted. 'This is deadly serious. The scholars are working out the trajectories of an arbalest bolt. We are developing an aiming system, you see. And the boy with his counters on the board is figuring the numbers for the scholars as they call out the sums.'

Orm frowned. 'I don't see any numbers.'

'But they are here nonetheless, represented by the beads in their columns. This is called an abacus, Orm. It's a counting system. You can add, subtract. You can even multiply numbers together with ease.'

Orm scoffed. 'Everybody knows you can't figure numbers beyond nine hundred.'

'Using this, you can go as high as you like. With such gadgets a ten-year-old Moorish child can count better than the King of England. I'm not surprised you haven't heard of this, or of Arabic mathematics in general. Mark my words, one day everybody in Europe will be counting this way.'

'Turning prophet again, Sihtric? Well, we won't be around to see it, one way or another.'

'True. But it's this sort of learning I came here to discover, and to exploit. Ah, here we are. My copy of Aethelred's original sketches.' It was a well-thumbed compendium of parchments – a document Orm hadn't seen for twenty years, since the day he had met Aethelmaer in Westminster. 'The Engines of God…'