When it was her turn, Mardina followed Quintus and Chu and a handful of Romans, and passed through a portal into a box of glass, a box riding on upright rails, which in turn were attached to a tremendous vertical wall that stretched above and below her, as far as she could see. Behind her in this glass box, Ruminavi the apu settled on a seat, surrounded by a handful of spidery axis warriors, and the Romans crowded in. And ahead of her …

She recoiled from the view, closing her eyes. She heard a kind of moaning, high-pitched, like a frightened animal. She thought it might be Chu Yuen, the slave, more intelligent than the average legionary and therefore more capable of wonder, and horror. She hoped it wasn’t herself.

‘Look down,’ the ColU said now, from the security of its lodging in Chu’s backpack. ‘Mardina Eden Jones Guthfrithson, listen to me. Don’t look ahead, or up – don’t look at the wall to which we are fixed – just look straight down.’

Mardina opened her eyes and looked. And, through the transparent floor, she saw what looked like Terra as seen from low orbit, a slice of sprawling landscape, washed-out green and grey under scattered clouds, and with stretches of water that glistened in the sunlight like polished Roman shields. ‘This isn’t so bad,’ she said with relief.

‘Here at the axis of the habitat we are over two hundred miles above this landscape. For that is the radius of this cylinder. The view here is just as if you were in a spacecraft, orbiting.’

‘It seems almost normal, in the sunlight. Except—’

‘What sunlight?’ the ColU said. ‘I know. There are breaks in the habitat’s tremendous walls. Pools that admit what must be reflected sunlight, to illuminate this enclosed environment – surely indirectly reflected, so that the radiation shielding is not compromised. There is one below us and not far ahead – you can look up now, just a little further …’

The sunlight pool glared under the clouds, like a city on fire. It was an eerie, beautiful sight.

Ruminavi said, ‘We call them the windows of Inti. For Inti is our sun god, you see.’

The transport suddenly lurched into motion, heading down the face of the wall on its rails. The passengers were jerked into the air, like pebbles in a dropped helmet, Mardina thought, and forced to grab onto whatever handholds they could reach. Already some of the legionaries looked as if they wanted to throw up.

Ruminavi, safe in his seat, looked on with a malicious grin. ‘Keep tight hold. The acceleration will be high. We’ll be covering a lot of your Roman miles every hour by the time we hit the atmosphere. Of course by then you’ll be feeling the spin weight …’ He laughed out loud. ‘Not so tough now, you Romans, are you? Just like your ancestors who begged on their knees to Tiso Inca’s generals to spare their city from the Fist.’

Quintus Fabius glared at him.

‘All right,’ the ColU said now. ‘Look down again, Mardina Eden Jones Guthfrithson. And look up. Look at the wall itself, down which we are climbing …’

It was more than a wall, she saw now, it was an engineered cliff face, crusted with structures, blocks and domes and pyramids – many essentially constructed of steel, Mardina thought, but ornately painted, even faced with stone and bound by steel straps. Structures – that was the wrong word. She saw lights gleam from within, doorways opening: these were buildings, inhabited by people. At the axial point itself a tremendous tower sprouted straight out from the wall, built of stone blocks of some kind: a stepped pyramid, skinny and enormously long. And in one place she saw a gang of workers, in pressure suits, tethered to handholds fixed to the wall, engaged on the construction of something new. A living, changing place then, a vertical town, stuck to this wall. And the rails on which the transport ran cut through all this clutter in a dead straight line before plunging down into the clouds far below.

Mardina uttered a silent prayer. ‘It is a city in the sky.’

‘No,’ said the ColU. ‘A city above the sky. We are in a near vacuum here, Mardina. The air will only become significantly dense perhaps twenty miles above the ground – I mean, above the cylindrical hull. This habitat, four hundred and fifty miles in diameter, essentially contains a vacuum, with a thin layer of air plastered over its inner surface, kept there by the spin gravity.’

‘A vast city in the vacuum. Why’s it here?’

The apu snorted. ‘Why do you think? This is Hanan Cuzco, home of the Inca himself, and his family and heirs and closest advisers. The greatest marvel in Yupanquisuyu, outshining even that dump Hurin Cuzco at the eastern pole. The mitimacs are kept out by all this lovely vacuum. Why, a war could be raging down there on the ground and we’d never know about it up here.’

‘ “We”, Ruminavi?’ said Quintus. ‘But you don’t live here, do you? It was my understanding that you’re coming with us, all the way to this grubby antisuyu, where you live.’

Ruminavi scowled. ‘Yes, and let’s see how long your Roman arrogance lasts in my jungle, you posturing clown.’

Mardina looked again at the compartment’s rear wall, the relatively comforting vision of a riveted metal wall flying up past her face. Hundreds of miles of metal, of steel and rivets … ‘All right, Collius. I think I’m ready for the next stage.’

‘Very well. Stay upright, feet down towards the ground – so to speak. When we are further from the axis the spin gravity will become stronger and pull you down. Now look straight ahead, lift your face slowly …’

If she had been in orbit around Terra, at this altitude the curve of the world would be apparent; she would find a horizon in every direction she looked. But here it was different. Here, when she lifted her head, she saw the panorama below her, of rivers and hills and inland seas and what looked like farms, what looked like cities, extending directly ahead, the details becoming a compressed blur with distance, until at last she saw only a band of air glowing with the illumination of the light pools. There was no sense of curvature – not if she looked straight ahead. But if she looked away from that axis, the landscape curved up, rising to either side and joining over her head to form a tube of smeared light, green and blue and grey. It was as if she was holding a rolled-up map, she thought, and peering through it at a distant source of light.

Far away, at least, it was all a comforting abstraction. But then she let her gaze wander back down the length of the tube, back to her position, and she looked up at a great roof of land, plastered with inverted mountains and patchwork farms and even rivers, pinned there by a spin weight she could not yet feel. She felt her heart hammering, her breath growing shallow.

The ColU said, ‘Easy, Mardina. Chu Yuen – hold her hand.’

The touch of the former slave’s flesh was comforting. But, glancing to her side, she saw that Chu had his own eyes clamped shut.

She laughed.

‘Are you all right, Mardina?’

‘Yes, Collius. A folded world. What magnificence. What arrogance. What madness!’

‘Quite. Yet here we are. Chu Yuen? What do you think?’

‘That I miss the stars,’ the slave said. ‘But I am now, however, standing on the floor of this box.’

He was right. Mardina hadn’t noticed. She was light as a feather still, but when she jumped, she drifted back down.

Ruminavi said, ‘Some way to go yet before we descend into the clouds. But we are already a tenth of the way there, and so you have a tenth your weight. We carry no refreshments, save water from that spigot over there …’

Mardina glanced around the transport, aware of her companions for the first time in a while. As their weight returned, the legionaries were pulling off their boots and settling down on their cloaks and blankets. Titus Valerius was playing knucklebones, or trying too, complaining loudly about the way the pieces rolled in the low weight. The medicus was huddled in a corner, obviously trying not to look terrified. One of the soldiers seemed to be taking a nap.