“Let’s keep going,” she said shakily.

“Yes,” said Crier, and fell silent once more.

The pitch blackness of the deep underground never returned, but it did grow darker as they walked what had to be a trail parallel to the river, leading them to the fortress itself. Finally, out of the darkness: a flickering light came from up ahead, like a faraway hearth fire. Crier’s pace quickened and Ayla followed, desperate to leave the dark behind. The light grew stronger and stronger, turning from deep red to orange to hearth-flame yellow, shadows scattering and reforming on the walls. The closer they got to the light, the more Ayla could hear . . . a thresher? A mill? Tumbling water, clanging metal, a whip-crack splintering noise like ice cracking on a frozen lake. She hadn’t heard that sound since she was a child in Delan, at the edge of the ice fields.

“I think we’re getting close,” she whispered, and of course that was when they hit a dead end. Or—not. The tunnel was blocked off by an iron gate, but when they crept up to the gate, when they peered through the bars . . . It seemed they were stationed high up on a wall, overlooking a big underground chamber. And gods, it was crawling with Automae. Watchers. They were dressed in the thick, protective leather Ayla had seen blacksmiths wear, their faces masked. Thick white smoke billowed from the center of the cavern, where Ayla could see leaping flames and a row of huge metal vats, like the heartstone cauldrons in the kitchens of the sovereign’s palace but triple the size. The smoke stung her eyes.

“This has to be the forge,” she whispered. “The heart of the Heart. They’re making heartstone.”

Crier opened her mouth to reply, but then her eyes widened. “Someone’s coming,” she hissed.

They had no choice but to retrace their steps, racing back down the tunnel, even though the Watchers would be coming from the same direction. Ayla kept her eyes on the tunnel walls, looking for any sort of escape route, even a fissure in the stone—and then she saw it. A round, hatch-like door. She grabbed Crier’s sleeve and dragged her over, praying it would be unlocked, praying there wouldn’t be more Watchers on the other side—

They tumbled through the hatch into a bloodred room.

Ayla blinked, disoriented. The room was lined with wall sconces, candles flickering behind red glass, casting a sickly, bloody light. For a moment, ridiculously, Ayla thought they’d stumbled into the Watchers’ sleeping quarters. There were two rows of little white cots, exactly like the cots in the servants’ quarters of the sovereign’s palace. And they were occupied. Twelve sleeping bodies. Ayla tugged at Crier’s sleeve—they had to get out of here, what if the Watchers woke up—

“No,” Crier whispered. “Oh, no.”

Then she was moving toward one of the cots. Have you gone mad? Ayla wanted to say. She scampered after Crier, prepared to drag her out of here by force if necessary, but . . .