Exhausted, filthy, uncomprehending, I peered up. About a hundred paces across, it was just a rough pile of mud bricks. And yet it towered above me, reaching up to infinity.

L’Eesh was breathing hard, sucking water. ‘Magnificent,’ he said. ‘Mad. They built a brick tower to reach to heaven! . . .’

I went exploring.

I came to a crumbled gap in the base of the tower. I crawled into an unlit interior. My suit’s low-output bioluminescent lamp glowed. I craned my neck. The bridge rose up vertically above me, a tunnel into the sky.

Metal gleamed amid the rubble on the floor.

I kicked aside half-bricks and uncovered a squat cuboid about half my height. It was featureless except for a fat red button. When I pressed the button the cube rose magically into the air, trailing a rose-coloured sparkle, like the bogeys’ vacuum-energy weapon; I kept out of the way of the wake. When I released the button the cube dropped again.

It was pretty obviously a lifting palette.

There was another palette buried in the wall of the bridge - and further up another, and another beyond that.

‘Now we know how they made their castles fly,’ L’Eesh said. ‘And how they raised this bridge.’ He was standing beside me, his suit glowing green. I saw he had scraped a channel in mould-softened brick with his thumb. Beneath it, something gleamed, copper-brown. ‘It’s not metal,’ he said. ‘Not even like Xeelee construction material.’

‘Maybe that’s the original structure.’

‘Yes. No suite of moons is stable enough to allow the building of a brick bridge between them; the slightest tidal deflection would be enough to bring it tumbling down. There must be something more advanced here - perhaps the moons’ orbits are themselves regulated somehow . . . The bridge itself is just a clumsy shell. The inhabitants must have constructed it after the intervention.’

‘What intervention?’

He sighed. ‘Think, child. Try to understand what you see around you. Imagine millennia of war between the two moons.’

‘What was there to fight over?’

‘That scarcely ever matters. Perhaps it was just that these were sibling worlds. What rivalry is stronger? Finally the moons were ruined, serving only as a backdrop for the unending battles - until peacemakers sent down blood-red rays, vacuum energy beams that turned the weapons to dust.’

‘Peacemakers? Silver Ghosts?’

‘Well, it’s possible,’ he said. ‘Though it’s not characteristic of Ghost behaviour. It was a draconian solution: a quarantine of technology, the trashing of two spacefaring civilisations . . . How arrogant. Almost human.’

I felt uncomfortable discussing Ghosts with human-like motives. ‘What about these lift palettes?’

‘It makes a certain sense,’ he said. ‘From the point of view of a meddling Ghost anyhow. A simple technology to help the survivors rebuild their ruined worlds - something you surely couldn’t turn into a weapon - but it didn’t work out.’ He smiled thinly. ‘Instead the populations used the gifts to build this insane bridge.’