The further away it was seen from, the more impressive the Glasshouse appeared. From the wooded top of Flag Hill, looking down across two rivers, the railways, the skyrails and four miles of grotesque urban sprawl, the facets of the dome glinted with clean shards of light. From the surrounding streets, however, the multitude of cracks and dark spaces where glass had fallen in were visible. The dome had been repaired only once in its three centuries of existence.

From the base of the dome the age of the structure showed. It was decrepit. Paint curled in great tongues away from the metal-work, and rust had eaten it like worms. For the first fifteen or so feet above the ground, the panes-each nearly seven feet square at the bottom, descending in width like pieces of pie as they approached the vertex-were filled with the same crumbling, painted iron. Above that, the glass was dirty and impure, tinted green and blue and beige in chance patchwork. It was reinforced, and was supposed to be able to support the weight of at least two decent-sized cactacae. Even so, several panes were broken and empty of glass, and many more were laced with a filigree of cracks.

The dome had been built without much concern for the surrounding houses. The pattern of streets that surrounded it continued until they reached its solid metal base. Those two or three or four houses that had been in the way of the dome’s edges had been crushed, and then the rows continued beneath the glass canopy, at a variety of random angles.

The cactacae had simply enclosed an existing clutch of New Crobuzon streets.

Over the decades, the architecture within the dome had been altered to adapt the once-human houses to cactacae tenants, tearing down some structures and replacing them with strange new edifices. But the broad layout and much of the structure was said to remain, exactly as it was before the dome existed.

There was one entrance to the dome, at the southern tip of its base in Yashur Plaza. At the opposite end of its circumference was its exit on Bytrash Street, a steep road that looked down onto the river. Cactus law stated that entrance to and exit from the Glasshouse was only by these portals respectively. This was unlucky for the cactacae who lived just outside in sight of one or other of the portals. Getting in, for example, might take two minutes, but returning home from the exit would involve a long, tangled walk home.

Each morning at five the gateways were thrown open, onto the short enclosed passage beyond, and each evening at midnight they were closed. They were guarded by a small unit of armoured guards, hefting huge war-cleavers and the powerful cactacae rivebow.

Like their dumb, rooted cousins, the cactacae had thick, fibrous vegetable skin. It was taut and punctured easily, but it healed fast, in ugly, thick scars-most cactacae were covered in harmless ganglions of scab tissue. It took a lot of thrusts or a lucky shot into the organs to have any real damaging effect. Bullets or arrows or quarrels were usually ineffective against cactacae. Which was why the cactus soldiers carried rivebows.

The designers of the first rivebow had been human. The weapons had been used during the ghastly premiership of Mayor Collodd-they had been carried by the human guards of the mayor’s cactus farm. But after the reforming Sapience Bill dissolved the farm and granted cactacae something approaching citizenship, the pragmatic cactus elders had realized this would be an invaluable weapon to keep their own people in line. Since then, the bow had been improved many times, this time by cactacae engineers.

The rivebow was an enormous crossbow, too large and heavy for a human effectively to operate. It fired not bolts, but chakris; flat metal disks with serrated or razored edges, or metal stars with curved arms. A toothed hole in the chakri’s centre slotted neatly onto a little bud of metal that emerged from the rivebow shaft. When the trigger was pulled, the wire in the shaft snapped violently to, pulling the metal bud at massive speed, intricate gears grinding together to send it spinning at an enormous rate. At the end of the enclosed channel the whirling bolt slipped sharply down and out from the chakri’s hole, and the chakri was discharged as fast as a slingshot stone, spinning like the blade of a circular saw.

Aerial friction dissipated its momentum quickly: it did not have nearly the range of a longbow or a flintlock. But it could sever a cactus limb or head-or a human one-at nearly one hundred feet, and slice savagely some way beyond that.

The cactacae guards glowered, and swung their rivebows with surly arrogance.

The late rays of the day blazed out from above the far-off peaks. The west-facing aspects of the Glasshouse dome glowed like rubies.

Straddling a corroded ladder that swept up to the peak of the dome, a silhouetted human figure grasped and clung to the metal. The man crept gradually up the rungs, rising up the curved firmament of the dome like the moon.

The walkway was one of three that extended at regular intervals from the very top of the dome’s arch, designed originally for the repair crews that had never appeared. The curve of the dome seemed to break the surface of the earth like the tip of a bent back, implying a vast body below ground. The figure was riding a gargantuan whaleback. He was buoyed up by the light that the dome trapped, that played on the underside of the glass and made the whole great edifice shine. He kept low, moving very slowly to avoid being seen. He had chosen the ladder on the Glasshouse’s northwestern side, so as to avoid the trains on the Salacus Fields branch of the Sud Line. The tracks passed close by the glass on the opposite side of the dome, and any observant passengers would see the man crawling up the curved surface.

Eventually, after several minutes climbing, the intruder reached a metal lip that surrounded the apex of the great structure. The keystone itself was a single globe of limpid glass about eight feet in diameter. It sat perfectly in the circular hole at the dome’s apogee, suspended half in, half out like some great plug. The man stopped and looked out over the city, through the tips of the supporting struts and the thick suspension wires. The wind whipped about him, and he clung to the handholds with vertiginous terror. He looked up into the darkening sky, the stars dim to him from all the clotted light that surrounded him, that ebbed through the glass below his body.

He turned his attention to that glass, scanned its surface minutely, pane by pane.

After some minutes he raised himself and began to climb backwards down the rails. Down, fumbling with his feet, feeling for holds, gently probing with outstretched toes, pulling himself back towards the earth.

The ladder ran out twelve feet from the earth, and the man slid down on the grappling hook he had used to get up. He touched the dusty ground and looked around him.

“Lem,” he heard someone hiss. “Over here.”

Lemuel Pigeon’s companions were hiding in a gutted building at the edge of a rubble-strewn wasteground flanking the dome. Isaac was just visible, gesticulating at him from behind the doorless threshold.

Lemuel paced quickly across the thin scrub, treading on bricks and concrete overgrown and anchored with grass. He turned his back on the early evening light and slipped into the gloom of the burnt-out shell.

In the shadow before him crouched Isaac, Derkhan, Yagharek and the three adventurers. There was a pile of ruined equipment behind them, steam-pipes and conducting wires, the clasps from retort stands, lenses like marbles. Lemuel knew that the mess would resolve itself into five monkey-constructs as soon as they moved.