The framework was wrought in black iron, a great thick skeleton decorated with occasional curlicues and flourishes. It bulged out over the Riverskin houses, visible from a long way off on the top of its low hill. Emerging in two concentric circles from its skin were colossal girdered arms, nearly the size of the Ribs, suspending the dome and taking its weight on great cords of twisted metal.

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 metalwork, 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.