But the staircase itself kept on going up. It didn’t stop at the roof, where by all rules of logic and common sense it should have done, but continued its impossible climb. A cool breeze drifted down, laden with the smell of water and stone.

The Carceri, Irene thought. We must be at the border.

‘Identify yourself,’ the first guard said. ‘If you’re carrying authorization, show us, but keep your movements slow.’

Bribery or intimidation wasn’t going to get them past these guards - they were alert and professional. Beguilement was a possibility, but this time there was a simpler resolution. Irene turned her head till her lips were against the protective iron wall of the staircase and murmured, ‘Iron panels, close to encircle the staircase and block all entry from outside it.’

The metal seemed to scream as it moved, creaking and straining against the frames of the outer panels and the rods that held it in place. The sides of the stairway warped out of shape, forming a barrier between them and the guards, wrenching the interior designs of the openwork panels out of true and twisting them into scrap.

But it was protective scrap. Vale jolted into movement, scrambling further up the stairs - past the guards in their rooftop space and into the part that shouldn’t even exist. Irene was just a step behind him.

One of the guards sprang into action, firing his pistol. The bullet bounced off a panel, cracking against the stone wall. Another guard had more sense and ran round the staircase until he found a gap between two warped panels, thrusting the muzzle of his pistol in and aiming it up towards Vale and Irene. The bullet winged Vale’s upper arm and rang off the iron post at the centre of the staircase, then fell to rattle down the steps in a succession of pings. Blood spattered and Vale cursed, clutching his arm, but they kept running.

The staircase broadened impossibly as it rose and they left the furious guards behind. In practical terms they should have moved past the roof of the building by now, but the staircase kept on climbing. The walls were further away now too, barely visible in the darkness, with only the outlines of large blocks of stone being clear.

Irene wasn’t sure where even this small amount of light was coming from. She decided not to think about it, except to hope it didn’t vanish. Climbing a fragile wrought-iron staircase at an unknown height in near-darkness with guards somewhere below them was bad enough. Climbing it in total darkness would be even worse.

A heavy gust of wind came down the staircase, making the metal creak and shiver.

‘It’s getting darker,’ Vale called back over his shoulder.

Irene really wished he hadn’t said that. ‘Maybe they usually bring lanterns,’ she answered. ‘How’s your arm? Is it bad?’

‘Merely a flesh-wound,’ Vale said dismissively. ‘Can’t you use your Language to seal it?’

‘You’re a living thing,’ Irene explained, in between panting for breath. ‘I can tell it to close, but it won’t necessarily stay shut. I’d need precise anatomical knowledge to hold it together. Bandages are going to be of more use.’

And then it was getting light again. The walls were far away now, and the staircase was a single metal spiral in the middle of a frighteningly large space. There was still no clear way to define from where the light was emerging. When she looked out through the spaces in the iron panelling, Irene could see distant walls and a far more distant ceiling, but no sky or artificial lights. She was breathing heavily now, and her legs were aching.

Another stronger gust of wind made the staircase tremble again. This time both she and Vale slowed their pace, and she saw Vale’s hand tighten on the central post as he steadied himself. Blood streaked his sleeve and had spattered across his jerkin.

‘Wait here,’ Irene said firmly. ‘We’ve enough light. I need to bandage your arm before you lose any more blood.’

Vale peered through the panelling. ‘I can see something a little further up. Perhaps we should reach that first?’

‘If there is danger up there, I’d rather we stopped your loss of blood before we run into it.’

‘Oh, very well,’ he said pettishly and sat down on the stairs, bracing his arm on his knee. ‘It doesn’t feel that serious.’

Irene wasn’t sure whether to ascribe that to a casual disdain for injuries - being shot might be an occupational hazard - or simple unwillingness to admit weakness. Rather than get into an argument, she sat down beside him and peeled back his sleeve. A thin, sluggish line of blood oozed down from where the bullet had ripped through the muscles of his upper arm. ‘You’re lucky,’ she said calmly. ‘It didn’t hit an artery.’

‘I would certainly have noticed if it did,’ Vale muttered.

‘Do you have any brandy on you?’

‘No. But I doubt we’ll have time for it to go septic in any case.’

Time, yes. Time was a voracious clock eating up the minutes and forcing them closer and closer to disaster. ‘Excuse me,’ she said, bringing out her knife. A few seconds’ work turned Vale’s blood-stained sleeve into a couple of pads, one for each side of the arm, and the bottom of her skirt was repurposed into a bandage.

Vale looked at the ungainly wad of fabric. ‘Did you ever train as a nurse, Winters?’ he said through gritted teeth.

‘Only basic first-aid and life-saving. You know, sprains, fractures, bullet wounds, sulphuric acid, that sort of thing.’ She tucked her knife away. ‘I wonder if there’ll be more guards at the top.’