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Layer six is different, and I know it before we reach the plate.

The challenge has the usual two parts. What is not usual is a third field, rendering as a tolerance, and this one applies to the inputs themselves.

"Simultaneity," Claire says, gone completely still. "Both answers inside four hundred milliseconds of each other."

I read it twice. "You cannot coordinate that."

"You can't coordinate it at all. No countdown either — it rerolls every four seconds. You'd need two people arriving at different answers to different questions who put their hands down in the same half-second without a signal." Her head comes round slowly. "That's why two layers cost them eleven days. Their pair passes the examination. Their pair cannot pass this."

I stand in a hot dark corridor and understand at last what the builders were protecting themselves against.

Not thieves. Not soldiers. Committees. Contractors. Two clever people hired to be clever at a plate. Anybody can hire one mind that calculates and one that improvises. Nobody can hire two people who have spent long enough at the same problems to stop needing to speak.

"They can't get in," I say.

"Not the way we can." Claire flexes her fingers. "Which means we're first, and first alone, and once we're through they have one route left."

"Which is?"

"Us."

CHAPTER 29

CLAIRE

Opening the sixth gate is the strangest four seconds I have lived through, and I don't remember doing it.

I remember the challenge rerolling. I remember reading a fault condition and knowing, with total certainty, that a bypass through the auxiliary loop would hold it — and feeling Darius arrive at a shim value from a completely different direction at the same instant, both things true inside one head instead of two.

And then our hands are on the plates. Together. No count, no glance, no signal.

Three hundred and eleven milliseconds apart. The gate logs it.

Layer six opens onto a ring gallery around a vertical drop, and forty meters below sits a socket of cold blue fire with a machine in it that has been there since before anybody wrote anything down.

We have about eleven seconds to look at it.

Then the ambush, and it is a good one, and it works because they were never trying to open the gate. They were waiting for somebody who could.

Thirty of them out of the gallery's blind arc, and not the men from the bore. These are the ones Marr keeps for the end of things. Two flash charges. Tomas's people go one way around the ring and we go the other, and a ring is a circle, so no direction is away.

Darius has my wrist and then a wall of them comes between us and he doesn't.

Somebody hits me across the back of the head and the gallery turns over, and the last thing I have is Darius's voice doing something new, and the bond turning white with a rage that isn't mine and is entirely available to me, and then nothing.

I wake tied to a maintenance stool on the lower gallery with a headache like a structural fault.

Elys Marr sits four feet away on a crate, coat off, sleeves rolled, hands loose between his knees. Up close he looks tired. Nobody tells you that part. He looks like somebody who has worked very hard for a great many years.

"Architect Donovan." Warmly. "I've read everything you've written for nine weeks. The load-shed was extraordinary. Genuinely. I lost four men in layer two when the cooling went and I still thought so."

"You want me to authenticate the substitution."

"I want you to understand it first. You'll be less trouble if you do." He leans forward, unhurried. "You think I want to own the air. I want the air scheduled. Consider what this city has done for four centuries. Failed at random. Nine districts get water, four don't, nobody can say why, the Authority writes a report, and everybody suffers equally and unpredictably, which people mistake for fairness."

"So you broke it deliberately."

"I made the failures *legible*." No heat in it at all. "When Horizon fails a line, Horizon knows which line, when, and who is affected, and Horizon has the part in a warehouse forty minutes away. Nobody dies of surprise. Ask District Six whether they'd rather have a predictable shortage with a supplier or an unpredictable one with a machine."