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.