I raised myself on my elbows and peered over the edge of the bed, to see how Thomas fared on the floor. Anna had scolded me for leaving him there, lecturing that evil vapours lurked near the ground, but Aelric had found a straw mattress and the boy had seemed comfortable enough since then.

He was not there.

I rubbed my eyes and looked again. The blankets were thrown back, and there was a depression where he had slept, but of Thomas there was no sign.

I rose, and brushed past the curtain into the main room. Perhaps he had come to pick over the crusts of the last night’s meal.

He had not. Nor was Aelric there – his mattress, though recently used, was empty.

I was growing uneasy, but not yet overly concerned. For the past few mornings Aelric had left early to fetch bread from the baker; I supposed this time he might have taken Thomas with him. I pushed open the shutters on the front window, hoping to see some sign of them in the street.

The shutters did not give easily – the icy night must have frozen their hinges – but as they at last swung open I was dazzled by the crisp light which poured in. The entire street was turned white, drenched in a sea of snow as far as my eyes could reach. Nothing save the wind had stirred it, and from my high vantage it seemed as smooth as the marble floors of the palace. And as cold.

Only a single figure broke its pristine coating, a solitary man almost directly below my window. He wore a monk’s habit, but even in the chill of the morning he had pulled back his hood, so that the skin of his tonsure stared up at me. Breath steamed from his lips; he did not move, but seemed to be watching for something.

I stood for a moment as if the air had frozen my very soul. Was this the monk, I asked, the man who had contrived to murder the Emperor? Why should he be standing in the bleak dawn outside my house? But then, who else would be standing there? And Thomas was missing.

I shook free my amazement and ran to the girls’ room.

‘Helena,’ I said, ‘Zoe. Wake up. The man I seek . . .’

As my eyes adjusted to the gloom after the brightness of the street, my words fell away. One mystery at least had been solved.

‘Thomas! What in all Hell’s dominions are you doing here?’

He was sitting on the end of their bed, wrapped in a blanket and staring at me with wild, uncomprehending fear.

‘Helena! Is this your mischief? Are you mad?’ Outrage and urgency wrestled in my mind. ‘Never mind; we will talk on this later. A dangerous man – the man I seek – is outside our house, and I cannot let him escape. If Aelric comes and I am gone, tell him to follow if he can. And you,’ I said to Thomas, ‘get away from my daughters’ bed and cloister yourself in my room. I will deal with your wickedness presently. And yours likewise, Helena.’

Battling the confusion that raged within me, I pulled on my boots, grabbed my knife and hurtled down the stairs.

I came into the street and blinked; the monk was gone. Had I imagined him? No; I could see his footsteps in the snow, the trodden circle where he had waited, and two parallel lines where he had come and gone. I followed them with my eye and there, just at the crossroad, I saw a flash of darkness on the snow disappearing around the buildings.

With the chill air rasping in my throat, and my sleeping tunic no protection against the cold, I chased after him. Nothing stirred in the snowbound streets, and the tracks were easy to discern, if not to follow. The snow rose above my ankles, tumbling into my boots and trickling down so that my feet were numb and sodden. Even with the effort of forging a path my legs trembled with the cold, and I wished with a burning fervour that I had seized a cloak, perhaps some leggings, before leaving. But then I might have missed him, for those few minutes’ delay with Thomas and Helena had given him a start which I could not close, and for the first half-mile I barely saw him save in fleeting seconds before he turned another corner.

Mercifully, he did not make for the heart of the city, where the marks of others might have obscured his trail, but seemed instead to aim for the walls. Up winding alleys and treacherous stairs I followed him, sliding and stumbling where the driven snow masked hard contours. Forgotten washing, frozen like lead tiles, hung on taut ropes above me; but no-one appeared at the windows to haul them in. It was as if the winter storm had stilled the entire city, all save me and the man I chased.

The silence thawed as I came suddenly onto the Adrianople road. A few bold travellers ventured along it, mostly on horseback, but I had seen the monk turn west and now, with the snow thinner and the way straighter, I could lengthen my strides and close my pursuit. For vital seconds I was unseen and unnoticed, but then the monk cast his eyes back over his shoulder, saw me, and began to run. I tried to increase my pace still further, but there was little purchase to be had on that road and my legs were already stiff with cold. Thankfully the way was wide and straight, so there was no losing the monk, but he remained as far beyond my reach as ever. We careered through the trickle of traffic, kicking up plumes of snow behind us, though there was nothing I could summon to gain on him. But soon we would be at the walls, and then he would be trapped. He must have realised this, for at that moment he veered suddenly right down an alleyway. I flailed my arms to keep my balance as I followed him, but too late – he had vanished. I cursed my luck, and his wiles, but did not succumb to misery, for the snow was thicker again and his tracks were fresh.

And then, it seemed, he flew away, for ahead of me the tracks stopped abruptly in the middle of the road. I came nearer and nearer, looking about for fear that he might have leapt into a doorway to ambush me, but he would have needed a giant’s stride to make that leap and there was nothing. Was there another Genoese invention which would carry men into the air?

I reached the end of his trail and understood. He had vanished not into the air, but into the ground: the footmarks finished at a narrow hole, a dark circle in the spotless snow. The iron disc which had covered it lay discarded a little way away, and at its rim I could see the first rung of a ladder leading down. From the bottom, perhaps thirty feet below, the mirror-gleam of black water told me it was a cistern.

A more cautious man might have waited there for help, for men with swords and torches to flush out the monk like a hunted boar. But the blood was flowing quick under my skin, and I did not know how many other tunnels might lead out from the chamber. Barely thinking, I lowered my legs into the hole and slid down the ladder. My palms burned with heat and splinters from the coarse wood, but I did not dare descend more slowly for fear that the monk might lurk at its foot, might drive a blade through me as I came down on him. When I could see the water was near, I pressed my foot against a rung and vaulted out into the darkness. The searing chill of the water clenched around me and I howled; had the monk been there he could have felled me at a stroke, for I was frozen in the icy water. I feared I might never move my limbs again so tight was its grip. My scream echoed around the dark hall, resounding off the domed ceilings and ranks of columns whose dim edges I could see in the pool of light shining through from above. Then there was silence. And then, at once some way off and all around, a frantic splashing.

The monk, I thought, and that sound stirred enough within me to lift my legs and start pushing through the chest-high water. I did not move quickly, but it took only seconds to leave my well of light and pass into utter darkness.

Was this how Jonah felt in the belly of the whale? I struck out blindly and felt a low wave ripple away from my chest, then slap against the surrounding forest of columns. One by one my senses deserted me: first sight; then sound, as the rushing echoes overrode each other in my ears; then, as the water numbed my soul, touch. I scraped against pillars and their pedestals and barely noticed, though the rough stone tore my shrivelled skin. Once my hand brushed something cold and clammy, and I started with a shout, but it was only a fish carried deep under the city by the aqueduct. I wondered if I had any more chance of escape than he.

Too late, I realised the futility of my ambition. I would not find the monk down here. Even with a score of men and fires there would have been endless columns for him to duck behind; alone, and in the dark, it was hopeless. Now my only thought was to escape, to be out of these depths and back in the light. I spun around, feeling the water swirling about my legs, and searched desperately for that beacon of daylight where I had entered.

‘Deliverance is of the Lord,’ I mumbled through shivering teeth. ‘Deliverance is of the Lord. Out of the depths have I called thee, Lord; hear my prayer.’

I thought I could see a smear of pale light somewhere to my left, surprisingly closer than I had expected. Had I stumbled around in a circle?

‘Christ have mercy. Christ have mercy. Christ have mercy.’

I repeated my prayers with a ferocity I had not felt since my days as a novice, and with each ‘Christ’ I forced another step forward. Soon the saviour’s name was little more than a whisper, a puff of air hissed through frozen teeth, but still it drove me on. The light was near now, cold and silent and beautiful, and I stumbled towards it with new hope. I could see the rungs of the ladder, shining like steel where the daylight met them; I could see the small circle in the roof where the world awaited. And there, far beneath it, I could see a dark figure hauling himself out of the water. His wet robe clung close about him so that he took the form of an eel or a serpent, the wet fabric gleaming like scales; the limbs which he stretched upwards seemed to be webbed into his body. I gave a faint, gurgling cry, and plunged forward, splashing and flailing to reach him before he escaped.

Even with the bewildering echo he must have heard me come, for I saw his head swivel round, and then his arms jerk up in frantic motion. I flung out a hand and felt it close around his foot; it pulled free of the rung as I fell back, but I did not let go. With a shriek and a howl the monk lost his grip, and there was nothing I could do to move as he came crashing down on me. His falling weight pressed me under and I sank, convulsing as my lungs drew in great gulps of icy water. I tried to stab him with my knife but my hand was empty: in the confusion I must have dropped it and never noticed.

And that was my last hope gone, for my enemy had found his footing now and was holding me under, waiting for the water to drown the life out of me. I did not have the strength to resist, and a few feeble kicks did nothing to dislodge him. I had been a thoughtless fool to think I could trap him in this cavern, and now I would pay the price of pride.

Calm descended. I ceased my struggle, and he must have been almost as drained as I, for he seemed content to hold me there and let nature take its course, without advancing the moment by further violence. I was suspended in the void; the waters closed in over me and the deep surrounded me; I could imagine that the fingers on my throat were nothing more than drifting weed. There are men I have spoken with, often after a battle, who claimed that in the moment of certain death they were transported to some earlier time in their lives, but I felt none of that: only a dull warmth creeping through my veins, a serenity in the knowledge that my struggle was gone, and soon I would be with angels. And Maria, my wife.

But not yet. Suddenly the hands which held me down drifted away. I was rising through the water, and could feel a stinging on the crown of my head where it was exposed to the biting air above. Then it was on my shoulders, my back. My body drifted and my foot touched ground; I pushed up, and gasped as my head broke free. No-one pressed it back. I gagged and choked, coughing gallons of liquid out of my lungs and trying to overcome the wracking pain which had exploded in my head. Somewhere, I thought, I heard someone call my name.

‘Demetrios. Demetrios.’

I opened my stinging eyes. It was not Maria, still less the angels. It was – against all hope and reason – Sigurd.

He lifted me out of that cave and slung me over his shoulder, pumping ever more water out of me as his armour rose and fell against my stomach. Dazed and bedraggled, I saw the snow-bound city turned on its head. He carried me tirelessly, never stopping, up stairs and twisting passages, across great roads, down narrow lanes and through stout gates, until I was brought within a room and laid in a bed. I shut my eyes, and the soft voices over me did nothing to spur my consciousness. Instead, I fell into a profound sleep.

I rolled over, opening my eyes further. I recognised this room, with its whitewashed walls and small windows: it was the hospital at the monastery of Saint Andrew, and by a chest a little distance away stood Anna.

She was not warm, not even remotely; she was entirely naked. She was brushing her hair, and the motion of the arm behind her head lifted her bare breasts like some antique statue. Her small nipples were puckered tight and hard, while by her hips the olive skin of her stomach rose gently as she breathed. Such was her lack of modesty that she did not even try to hide the dark shadow between her thighs.

For a moment I stared like some virgin on his wedding night; then, overcome with guilt, I belatedly pressed my blushing face into the pillow.

Anna laughed; a soft, forgiving laugh.

‘Come, Demetrios,’ she mocked me. ‘You were married, and raised two daughters to womanhood. Surely you must have uncovered these mysteries before. Am I so shameful?’

‘Shameless, I think.’ My humour returned a little, and I risked looking back. I was just in time to see her arms wriggling through the sleeves of a woollen camisia, which tumbled down over her body to mask its temptations. I felt an ache of regret that I had not looked longer, but dismissed the thought at once.

‘Do you always undress before strange men in the middle of the day?’ I watched her pull on her green dress and fasten the silken cord around it.

‘Only when they appear at my door half-frozen and close to death. I had to force some heat into you, so I lay beside you in the bed until you stopped shivering. You served in the legions – surely when you campaigned in the mountains you huddled together with your comrades at night?’

‘If we did, we kept our clothes on.’ I had endured much that day; it seemed almost too much to believe that I had risked mortal sin lying with Anna and not felt a moment of it.

Again I drove back my thoughts from the places they strayed. ‘And how did I come to be here?’

‘Sigurd brought you. He said he found you almost drowned in a cistern.’

‘Is he here now?’ Had he watched while Anna undressed and shared my bed?

‘He had important things to do. He said he would return, and try to bring some fresh clothes.’

Only now did I realise that under the blankets, I too was wholly naked. I pulled the covers closer.

Anna tied the scarf over her head and crossed to the door. ‘I must go. I have other patients to see. I will send an apprentice with some soup, and try to visit soon.’

‘Will you share beds with all your wards?’ I raised myself on one elbow.

The door closed without answer.

‘That’s the second time I’ve saved you from a battle you were foolish to enter,’ he said pointedly. ‘There may not be a third.’

‘I know.’ I was honest in my gratitude. ‘But how did you find me? And what of the monk?’

‘Your elder daughter found me with Aelric. I met him in the street; he had left his station to go and buy food.’ I did not envy Aelric explaining that to his captain. ‘We followed your tracks through the snow as far as the Adrianople road, where there were plenty of witnesses who could remember a bare-headed monk and a half-dressed madman chasing him. From there we searched the side-streets until we found you.’

‘And the monk?’

‘We saw him trying to drown you at the bottom of that hole, but as I came down the ladder he fled. I let him go; only a fool would follow a man into that abyss. My men are guarding the entrance. If he comes out, we’ll catch him.’ He looked theatrically at the sky, though the sun was veiled in cloud. ‘If he’s still down there, he’ll already be dead.’

‘We should go and see.’ I stood, feeling the trembling in my legs as they took my weight. I was weak, but the food which Anna had sent gave me strength, and the hunger to see the monk who had almost killed me was all consuming.

‘Will the doctor let you go?’ Sigurd asked with a smile. ‘She protects her patients like a tiger, you know.’

‘If you choose to risk your health and your strength running around the city, trying to do the monk’s work for him, then do so,’ she said briskly. ‘I need your bed for the more deserving anyway.’

Sigurd and I walked out of the monastery. It was late afternoon, and the road was almost solid with the humanity herded onto it. The snow, so pristine that morning, was now ground to a grey slush and mixed with grit and mud. It was well that the ground stayed frozen, or many might have sunk into an inescapable mire.

‘I must go to the walls first,’ said Sigurd. He had seemed cheerful at my bedside, but now his mood was grim. ‘I need to check on the garrison. The monk will wait an extra half hour – whether he’s under, on or in the ground.’

I did not argue, but pushed my way after him through the tide of men and beasts which flowed against us. It was straining work, and if I had not had Sigurd’s commanding bulk to follow I doubt I would have progressed a step. There was an intensity in the crowd now which I had not noticed previously: a hunch to their shoulders and a desperation in their gaunt faces. Perhaps it was the burden of snow and cold added to their already straitened condition, or perhaps they knew that the city was ill able to provide for them after the many others who had preceded them.

Sigurd had anticipated an extra half an hour, but it was almost an hour later, near dusk, when we at last reached the walls. Along them the Watch had kept a corridor free for messengers and heralds to gallop through, and I was glad of the space to breathe as we came into it.

‘My men are up that tower,’ Sigurd told me. ‘Will you wait?’

A squadron of cavalry thundered past, drowning my reply and spraying me with mud. Above me, a ballista was being winched up a tower on a scaffold, straining at the thick ropes which held it.

‘I’ll come up.’ I did not want to end that day crushed under a horse or a falling siege weapon.

As ever, Sigurd was recognised, and we were waved up by the guard at the foot of the stairs. It was not an arduous climb, but my head ached again and my legs begged for rest. About me, I could see sentries scurrying about, shouting and calling, though I could not hear what they said.

We came onto the broad rampart and my interest rose. A hush had fallen, and the guards were still, their faces pressed against the embrasures as if watching for a miracle. Sigurd ignored them and continued up the steps to the turret, but – drawn to the spectacle – I crossed to the battlements and stared.

Out across the snow-swept fields the sun had sunk beneath the rim of the clouds, facing us like a glowing eye. The sky and land alike were caught in its crimson glare, shimmering red, but that was not what had silenced the watchmen. On the ridge across the plain, some two miles distant, an army had appeared. They rode towards us with the sun behind them, their spears like pricks of flame and their banners dark above them. They were moving forward, but as one row passed into the shadows below the ridge another came up on their heels and took its place. It was a host of thousands – tens of thousands – and the snow turned black underfoot as they marched towards our gate.

The barbarians had come.