Erlan did as everyone else, looking away from the billowing dust-cloud and keeping his mouth clamped shut. Fortunately, the wind carried most of it over the walls to the south of them. But when he looked again and the dust started to clear, he saw some defenders staggering, clawing at their eyes, wailing in pain, some even throwing themselves over the wall to get away from the noxious fog that lingered around the teeth of the rampart. The worst of it had blown away but Erlan still tasted an acrid burn in his throat.

‘Bastards,’ the emperor muttered grimly.

Erlan remembered the horror of the Byzantines’ liquid fire. It seemed to him the quicklime was no crueller than being melted alive. But he kept the thought to himself.

‘I heard they threw beehives into Pergamon,’ said the fat eparch.

‘And baskets of snakes into Abydos,’ said another, wearing the red trim of a senator.

‘Please, sire.’ Katāros again implored the emperor to retire to a less exposed position. This time Leo conceded. His retinue shifted north along the wall and Erlan, Einar and the other imperial guards with them, descending a few dozen feet down from the summit of the hill. The emperor was right: they could see far less of the Arabs’ manoeuvres from their new position, but by now Erlan couldn’t believe that the enemy were going to make any headway with their siege weapons. The walls were simply too massive; the defences too deep. He wondered whether their commander, this Prince Maslama, would order a single concerted attack of his foot soldiers against one of the gates. But even if he did, there was no way it would succeed. The Byzantine defenders had ample supplies of projectiles to break the force of their momentum: great boulders, vats of boiling oil, bales of straw waiting to be lit. Any number of painful ways to check an Arab assault.

But the surest defence was the walls themselves. The gateways piercing inner and outer walls were deliberately misaligned. Even if the attackers forced entry through one perimeter, they would struggle to maintain the propulsion to carry them onward through the inner defences.

To be sure, defenders were dying – but not enough of them to cause the emperor any real concern. And Erlan found himself wondering what he would do in the place of this Prince Maslama. Would he waste a hundred thousand men on this futility? Does a man blunt his finest blade against a rock unless it serves him some purpose?

Perhaps it was with these questions circling in his head that his gaze drifted away from the main crucible of the fighting towards the north. There, the plain climbed, breaking up into craggy folds and gullies before it tumbled away again into the headwaters of the Golden Horn. Something in one of the folds caught his eye.

‘D’you see that?’ He gave Einar a nudge.

‘What?’

‘Something moving. Up in that gully.’