Ibrahim and Peter slipped out of Seville.

They came to a hole in the ground just beyond the city walls. It looked like the outlet of a broken sewer or drain. Peter said, 'This is older than the Moorish city – Roman, we think, part of their sewage system. Of course the settlement here was a lot smaller then. The main Roman town, Italica, was some distance away. It's a bit mucky down here-'

'Just get on with it.'

The hole in the ground turned out to be a shaft, deeper than Ibrahim was tall, down which he had to drop. He found himself in a stone-clad tunnel, too low for him to stand up straight. He could see no further than a few paces. There was a smell of damp and rot, but nothing foetid; the sewer was long disused.

Peter used a flint to light a candle. His eyes were pits of shadow. 'Are you all right? Not everybody is fond of the dark.'

Ibrahim took a deep breath. 'I have no love of being buried alive. But it's my mother I'm more frightened of.'

Peter laughed, and clapped him on the back. 'Come. Let's face our nightmares.'

It turned out to be only a short walk, though a clumsy and difficult one, through the low tunnel. Ibrahim stumbled over a broken Roman tile. Then the tunnel opened out, and Ibrahim found himself walking into a big boxy room. Steps cut into the earth led down to a floor some distance beneath him. The walls were stone-clad, the ceiling timbered, and lamps glimmered in alcoves.

And in this chamber, deep underground, machines brooded, dimly glimpsed. There was a great tube mounted on a carriage. An upright wheel turned, a treadmill, with a man inside it to work it. What looked like the skeletal form of a great bird's wing gleamed and creaked. Scholars and artisans moved among these creations, murmuring quietly.

Ibrahim felt deeply uneasy, as if he had descended into a sorcerer's pit.

Peter led him briskly forward. 'This was some kind of water tank,' he said in a murmur to match the subdued voices around them. 'Always built big, those Romans, even when it came to their plumbing!'

'I never knew this place was here.'

'Not many do. It's on no plans; I dare say your emirate doesn't know it exists. When we needed a place to work in secret your mother, ever resourceful, started asking around among the criminal element.'

'Criminal?'

'Smugglers. Hoarders. Even bandits. They knew of this hole in the ground. It wasn't hard to take it over, clean it up, extend it a little…'

'Ah, the vizier's advisor. How good of you to make time in your busy schedule to visit your mother.'

Ibrahim had not seen his mother for four years. Subh wore a robe, white and pristine despite the dirt, and her hair was piled elaborately on her head, jet black. Unlike Peter she showed not a trace of the passage of time; she was as upright, powerful and magnificent as ever. Peter seemed to cower before her; he was as much in her thrall as ever.

Ibrahim bent forward to embrace his Mother.

But she subtly moved back and offered her hand, cold, the palm oily. 'Let's keep things formal,' she said. She showed not a trace of emotion.

'Mother, you haven't changed.'

'And what of you?' she asked. 'You're clean enough. A smart costume. And well fed, it seems to me.'

'I take only my ration,' he said stiffly, and it was true, though there were many in the palace who did not.

She prodded his belly. 'In that case you're not getting enough exercise.'

'What are you doing here, Mother?'

'You know very well. Building the war engines that might save Seville. Walk with me. See what we have made…'

She showed him her marvels. Here was a metal tube that used compressed steam to spit iron balls. Peter called it the 'thunder-mouth', for the great roar it would make when it was fired. Around the perimeter of the treadmill he had noticed was a series of crossbows. An archer sat at the axle, and as the wheel turned one bow after another was brought before him.

'The archer only has to aim and fire,' Peter explained. 'See, the ingenuity is that the mechanisms of the wheel-engine load each bow for him as it turns. So this enables a much faster rate of fire than a conventional bow, without a loss of accuracy.'

There were many such gadgets, most only half-finished, betraying ingenuity but fragility.

Ibrahim refused to be impressed. 'This is all you've achieved, in five years?'

Subh watched him gravely. 'Don't you think anything of our efforts?'

He walked around the workshop. 'Your rapid-firing crossbow machine is vulnerable. A stick poked into the mechanism would jam it.'

Peter said, 'But a row of these machines, fixed to the city walls when the Christians come-'

'They would still break down. Men would do better.' He came to the thunder-mouth. 'This is more promising. More compact than a catapult, perhaps. Faster to reload and reuse. But it does no more than a catapult would.' He glanced around. 'I see nothing here which would give one side an overwhelming advantage over the other.'

Peter sighed. 'Well, you're right about that.'

'What we need,' Subh said, 'is Incendium Dei.'

'Your mysterious Fire of God.'

'Precisely. The fire that would turn these delicate gadgets into thunderbolts.'

'But you don't have it,' Ibrahim said.

'Joan of the Outremer never replied to my letter. And I regret writing to her now, for I told her something of what we have, without learning anything of her. I fear she might become a rival, not an ally.'

'Actually,' Peter said, 'it's not just God's Fire we need, Ibrahim. For these engines to be realised fully we need the original designs.'

'Ah,' Ibrahim said. 'The Codex. The treasure said to be buried under the floor of the great mosque of Seville. Is that why you asked me here? To get me to dig up the mosque?'

'No,' Peter said. 'I invited you here, frankly, because I thought you should be reconciled with your mother.'

'But now that you are here,' Subh said slyly, 'why not? You have the ear of the vizier. If you dropped a word-'

Ibrahim shook his head. 'You have buried yourself in this hole in the ground for too long. Think what the mood is like outside! In this crowded city the faithful wash around the muezzin tower like a sea. If I were to order the mosque floor to be dug up, in the hope of finding plans for super-weapons, I would cause a riot. And besides the imams would never give permission.'

'So you turn your back on us again,' Subh said, her tone poisonous.

'I regret what has happened,' he said. 'Nothing should come between mother and son.'

Subh said, 'But you still think I'm wasting my time down here, don't you? You're just as headstrong and unimaginative as you were as a boy.'

'Yes. I still believe you're wasting your time. And so, it seems, am I.' He turned to leave.

'If you won't help us,' his mother called after him, 'at least don't betray us. Don't let the emir put a stop to our work. Trust me that much.'

He paused. Then, without looking back, he made for the tunnel that led to the air, and the light.