It took almost an hour for Bleach-Hair to weave through the labyrinth of streets to reach his destination, but I could see his goal long before we got there: a towering assemblage of glass vats, arranged in a haphazard corkscrew around a central wooden framework that rose twenty storeys into the sky. Each circular vat measured ten paces in diameter and at least twenty feet high, filled with murky water and stocked with fish that skimmed relentlessly past the glass walls.

This was Sigil's famed Vertical Sea, a fish farm built long ago by a wizard named Churtellius: no doubt a master sorcerer in his day, but now only known for his love of seafood. He had painstakingly constructed each of the vats, strengthening the glass with magic so they could contain the weight of the water; he had personally supervised the raising of the support frame, designing the maze of ramps and trestles and catwalks so that the seemingly random arrangement of vats perfectly counterbalanced each other; and he had even laid out the complicated schedules for changing water in the vats, shoveling in fish food, and harvesting the catch for later sale at the Great Bazaar. Quite possibly, Churtellius had created the Sea in a spirit of purest charity, to ensure that Sigil had an abundant supply of fresh cod and salmon and scallops… but the chant on the street said Churtellius was just another barmy spellchucker who'd do anything to lock down a dependable supply of kippers.

Bleach-Hair went straight to the base of the tower, spoke briefly to the guards who watched the entrance ramp, then began making his way up the tall corkscrew structure. «Stay with him, Wheezle,» I whispered, though I had no idea if our friend gnome was within earshot. Quite possibly, he was already dogging Bleach-Hair's footsteps while Hezekiah and I lingered in the shadows of nearby buildings.

«Should we follow too?» Hezekiah asked.

«We're only here to watch,» I replied. «If we see evidence this really is enemy headquarters, we report back to Lady Erin and let her give these berks the rope. I for one am not spoiling to face a bunch of bashers with firewands.»

«Have you noticed,» the boy said, «when you get excited, you start to use words like berk and basher, the same as other folks in Sigil?»

«Pike it, Clueless,» I told him.

Hezekiah grinned from ear to ear.

Leaving the boy on watch near the base of the tower, I spent a few minutes roaming the neighborhood in search of a better view of the Vertical Sea. I found it at last in a tenement building across from the tower, much like the one we had used to observe the Mortuary, but with stairs leading up to the roof. Like most roofs in the Hive, it had a pathetically unproductive vegetable garden, several small chicken coops owned by various tenants of the building, and a crusty coat of bird droppings. I walked carefully across the guano, marking what an interesting squishy sound it made.

The smell was interesting too.

Crouching behind a chicken coop, I stared across the street toward the Vertical Sea. The tower was busy with people tending the vats – workers standing on catwalks above the water, netting up fish and dumping them into wheelbarrows, then trundling their loads down the ramps. Bleach-Hair pushed against the downward flow of wheelbarrows and continued to climb slowly. Since the last time I looked, he'd been joined by two familiar men: the other fireballers from the City Courts. Both of the newcomers held firewands in their hands.

Where were the three of them going? I scanned up the tower looking for anything out of place… and there, just below the level of my rooftop, was Yasmin.

Without the diligently developed eyes of a Sensate, I might not have recognized her. She wore drab work clothes now, and had smudged her face with soot. Nevertheless, her bony arm crests were clearly visible, and she still carried that sodding charcoal sketch I had drawn. In fact, she made a show of unrolling it from time to time, glancing at it, then rolling it up again, as if it was a scroll of instructions she was supposed to follow. The other fish-workers obviously accepted her pretense – they moved to and fro past her without a second glance.

Once I had recognized Yasmin, it was easy to pick out Oonah and Kiripao close by her side. Oonah still had her staff and Brother Cipher his air of serene lethality, but they too were disguised as workers, dawdling about with an empty wheelbarrow. I could only conclude the githyanki and githzerai had led my teammates to the Vertical Sea… and sure enough, as I looked farther up the tower, I saw the two thieves ambling along a ramp almost level with my rooftop.

They still wore their Dustmen robes, with hoods pulled down low. The clothes attracted attention from the regular workers, but probably not as much as the sight of a githyanki and githzerai walking amiably side by side. I watched as the two stepped off their ramp and onto a walkway over a vat of dogfish: scaled-down sharks averaging three feet long, with hungry looks in their eyes as they prowled behind the glass walls of their home.

I could see no immediate reason why the thieves would be strolling along a dead-end catwalk over a vat of fish; but as I strained my eyes, I saw that the struts supporting the next vat above their heads formed a sort of archway… and the arch was glowing.

«Well, I'll be piked,» I whispered. «It's a portal.»

Not that I should have been surprised to see a gateway to another plane halfway up the Vertical Sea. Throughout the multiverse, Sigil is known as the City of Doors; the place probably has more portals than rats, and Sigil has a lot of rats. Walk down any street, and you're likely to see a portal lurking somewhere – in the door to a bakery, along the covered cloisters of a temple, or even in the angle made by a ladder leaning up against a wall. Any sort of arch, no matter how temporary, can suddenly sprout a portal… and who knows if the portal leads to the blissful meadows of Elysium or the 500th level of the Abyss?

Of course, most portals are temperamental things; they refuse to work unless you're carrying the right «key». Suppose, for example, there's a portal anchored in the door of your neighborhood greengrocer: ninety-nine times out of a hundred, you could walk through and simply end up staring at the shop's supply of lettuce. However, if you happened to carry the particular class of object that activated the portal – a silver goblet, a triangular scrap of cloth, a rope with knots at both ends – the portal would magically wink open and deposit you somewhere else, a long way from home. If you passed through the doorway with a group of friends, they'd be sucked in too; open portals tend to be hungry.

Sigil's portals, blossoming by the hundred, formed the heart of the city's economy… especially among the local practitioners of magic. Some wizards, for example, worked on diagnosis; they detected new portals, divined what kind of key would make the portal open, and predicted where you'd end up if you passed through. Other mages specialized in prevention – for a hefty fee, they'd weave spells around your home to make sure the door into Great-Aunt Effy's bedroom didn't suddenly become a gateway to the Elemental Plane of Fire. A third class of sorcerers devoted themselves to understanding the whole portal phenomenon: what created them, why they worked, and how they chose what kind of objects would serve as keys.

That third bunch of sorcerers always went barmy in the end. There's no rational system to explain portals. They just do whatever they want… like anchoring themselves in an arch over a catwalk, ten storeys up the Vertical Sea.

The githyanki and githzerai sauntered along the wooden walkway, glancing casually around to see if anyone was looking their direction. Their gaze brushed past my hiding place, but didn't stop. When they were happy the coast was clear, they simply stepped forward and disappeared. From my position I couldn't see what lay beyond the gate in the brief moment it was active; but a thick sifting of dust puffed out of the opening, slowly settling toward the catwalk and the water surface below.