As soon as they entered the narrow streets, they lost sight of the mounted Turks, and everything else except the sun in the sky overhead.

They moved as fast as the bishop and the children would allow them to move. The men took turns carrying the children, and no one – not even Alessandro – proposed that the children be left.

Up Fifth Hill, and down again, with a quick glimpse of the Golden Horn off to the east, shining in the sun. Across at Galata, three low vessels were laid out on the quays, ready for sea.

Alessandro pointed to them. ‘That’s our ride home, my friends,’ he said, and the men responded with another burst of speed.

Down Fifth Hill on the eastern flank, into the dense slum at its base and up along the ridge that held both Fifth and Fourth Hill, headed almost due east. They took a looping series of alleys, and the priest apologised for the state of the streets.

Giannis kissed the man’s ring. ‘We would be dead without you, Father,’ he said.

‘See that your heretics save the head,’ he responded. ‘I need no other reward. Keep it from the filthy Turks, and perhaps some day it will return to us.’

High on the northern flank of Fourth Hill, Swan stopped the priest.

‘I want to get into the cisterns,’ he said.

The priest looked startled, although whether at his Greek or his knowledge of the cisterns Swan didn’t know.

‘I can see an aqueduct from here,’ Swan insisted.

The priest paused. ‘Yes,’ he admitted. ‘I can take you there. You know the sewers?’

Swan shrugged. ‘I think so,’ he said.

The priest nodded. ‘Few do. Most of them are thieves.’

The aqueduct ran above ground, well over their heads, but it gave Swan hope. He stowed the head again, in his armet, after the priest touched it, venerated it, and prayed.

Then they set off under the aqueduct, leaving the priest behind. Everyone was flagging. And the sun was headed for the horizon.

The Turks were fanning out. From the flank of Fourth Hill, it was easy to see that they had not found the embassy where they expected, and now the regiment of cavalry was breaking up.

‘Now is not the time to fail,’ Swan called. ‘Come on!’

The ground under the aqueduct was rocky, but there was an obvious walkway, and they jogged along, armour creaking and clanking as they went. The youngest Venetian cut away his tassets. They all drank water and ran again.

Down the ridge. Twice they had to detour around arms of the city, new or old, that had grown in under the aqueduct, and then they were north and west of the market, and Swan muttered a prayer of gratitude to God.

He knew where he was.

‘Almost there!’ he said. He pointed down into the market below them, where a ruined fountain had collapsed into the underground sewer.

Alessandro took him aside. ‘We’re spent. Is it true? We are almost to your underground palaces?’

‘Half a mile. Less. Please, everyone. We are almost safe.’

Alessandro stopped them, gathered them in a tight group, and they walked on, breathing hard – down the last slopes of the steep ridge, and into the back of a dense residential quarter, with houses packed tight – some abandoned and some very much inhabited. Greeks watched them with sullen indifference.

A group of young men began to shadow them.

And then, as they turned north to pick up the street that Swan knew led to the market, he saw Yellow Face at the same moment that the spy saw him.

The tall man turned, hiked up his kaftan and ran.

‘Kill him,’ Swan shouted at the marine, but none of them had a shaft on his bow, and Yellow Face was gone before they were ready to loose.

Alessandro looked at him, a question in his eyes and the set of his mouth.

‘A spy. For the Turks, I think. Perhaps for Omar Reis. Either way, they’ll be on us in a few heartbeats.’

‘Let’s run, then,’ said Alessandro, and suited action to word.

‘If they see us go into the sewers . . .’ Swan said. But he followed the capitano, and they all ran off down the street.

The bishop tripped and fell.

One of the sailors paused.

Swan tried to make himself run on, but he didn’t. He ran back and helped the sailor get the bishop to his feet. The man was bleeding from an abrasion on his chin.

‘Take this,’ Swan said, and handed his helmet to the sailor. He lifted the bishop, threw him over his shoulders, and staggered towards the market, now in sight.

A clod of earth hit the sailor. And he stumbled.

Swan thought, Why am I saving the bishop?

He staggered on.

A clod of earth hit him.

A young Greek man ran out into the street and yelled ‘Death to the heretics!’ in Greek, and another clod of earth hit – this time striking the bishop on his shoulders.

The weight of his armour and the weight of the bishop – thankfully, not the largest ecclesiastical size of bishop, but not a slim man – was cutting through his burst of spirit.

The oldest of the Venetian marines – the one that the others called ‘The Spaniard’ – turned, paused, and came back towards Swan. Without a word, he took the bishop’s legs, and they staggered on together, the sailor staying with them, eyes glazed with fear. He didn’t have a weapon.

Since no one came to drive them off, the Greek youths grew bolder, and there were stones mixed in with the clods of earth. They rang off Swan’s backplate and his arm harnesses.

Swan’s whole world narrowed to the effort of staggering, off balance, along the time-worn street towards the distant market. It didn’t seem to grow any closer.

He heard hoof-beats.

They started to cross a major thoroughfare and had only fifty paces to go to the market. The old ruin of the fountain was another fifty paces beyond. Swan looked to the left and saw the horsemen – three Turkish riders, with another dozen well behind them – coming at a gallop.

They were all but on top of him.

He and the Spaniard dropped the bishop in the street as the first arrow flew. It passed between them.

The second arrow screeched along Swan’s left shoulder, deeply marking the steel, and fell to the street.

The third arrow all but parted Swan’s hair and reminded him that he didn’t have a helmet. He got his sword out of his scabbard and his buckler off his hip. The bishop curled into a ball and prayed.

The first Turk hurtled by, an arm’s length away, leaning out over his horse on Swan’s buckler side, an arrow drawn all the way to his chin. Tom threw his buckler hand up as the man loosed, and the arrow struck his buckler’s steel boss and left a deep dent, all but numbing Swan’s hand.

The Spaniard didn’t wait for the second Turk, but stepped in front of his horse, severing the reins and slicing deeply into the horse’s neck – the horse was dead immediately and began to collapse under the Turk, who nonetheless took his shot at the range of a few feet. His arrow caught the Spaniard in the middle of the chest and knocked him down. Then horse and rider fell in a spectacular spray of dust and blood.

The third Turk changed direction to avoid the dying horse ahead of him, rose in his saddle, holding on with only his knees, bow drawn.

For what seemed like a brief eternity, Swan was looking down the length of that arrow, and then the Turk loosed. In the same heartbeat, the sailor holding his helmet lurched away from the dying horse and, tripping over the bishop, lifted the armet over his head. The Turkish arrow crashed into the Milanese helmet and careened away.

Swan saw the disgust on the Turk’s face as he went by.

The armet containing the head crashed to the earth.

The Spaniard was alive. The arrow had dented his breastplate and the man was struggling to breathe, but it hadn’t penetrated. The two Turks still mounted were turning their horses.

The other dozen were coming.

Swan gave the Spaniard his hand and lifted the man to his feet.