Before they reached the Forum of Constantine at the top of the hill, the litter turned left off the Mese and continued a short distance down a side street that must eventually lead to one of the harbours. There was some manoeuvring into a still smaller passage and at last she was set down. The door was held open for her and she got out. To her surprise, she found the emperor waiting for her, this time garbed in the simple clothes of a soldier, a thick cloak of red wool wrapped around him.

His greeting was effusive. ‘Good morning, Queen Lilla! Forgive this. . . well, I could say inconvenience but I assure you it will be worth your while. There’s something I have been wanting you to see.’

He led her into a nearby doorway which looked unremarkable enough. Inside were posted two of the city guard, one of whom passed Leo a torch, already lit.

‘This is a little mysterious,’ she said. Her Greek had much improved over the winter months; Leo’s best tutor had seen to that.

‘You will see why very soon.’

She enjoyed seeing him free of his purple robes. He seemed ten times more natural in the guise of a soldier, his movements deliberate, the hunch of his muscular shoulders strong, almost dangerous. He plunged through another doorway into darkness instantly lit by the torch flame.

‘Watch your footing. We go down now.’

‘Where are we?’ she asked, taking care to replicate his steps down a stone staircase that materialized out of the gloom.

‘This was once one of the city cisterns. It serves another purpose now.’

As they neared the base of the staircase, she had a sense of a deep looming space to her left, and as the flame-light chased away the darkest of the shadow, she could make out a small forest of pillars, each trunk as thick as a hundred-year oak, stretching away from her as far as the light would penetrate. They passed two more guards standing in the gloom at the foot of the stairs, a weak torch fixed to the wall there.

‘Imagine,’ he said, ‘all of this filled with water!’

‘Don’t you have need of it? The city cannot run dry.’

‘We have twenty more like it, and bigger, spread around the seven hills, all still nearly full. But this one –’ he turned, his smile a cave of shadow – ‘this has a very special purpose. Come – I will show you.’

She followed him deeper into the bowels of the cistern – which, devoid of water, seemed more like the underground hall of some dwarf king than a kind of well; and presently she noticed beyond the pool of torchlight another source of light ahead. There was a strong smell, sharp in the nostrils, and under it another, like the trace of rotten eggs. The other light soon grew brighter as they approached and then she heard voices and the light tap-tapping of metal; she saw torches sconced on the walls and pillars, and still further on a murky, green glow.