She glanced behind her but the mirror was gone. The dream—because this was a dream, she was sure, though unlike any she had ever walked—had sealed shut behind her like skin around a wound. There was simply more tunnel, running off into the darkness, and huddled shapes she originally mistook for animals until she realized they were people—mostly men—filthy and obviously homeless, gazing at her through alcohol-hazy eyes.

“You look like one of the lucky ones,” one of them rasped. She couldn’t tell if it was a man or a woman.

She turned around and hurried forward, toward the street. Her footsteps echoed on the stone, and made it impossible to hear whether she was being followed. She was too afraid to turn around, worried that if she did, she would find one of the men lurching after her. But she made it to the end of the tunnel, and when she finally risked a glance behind her, saw that she was alone.

Emerging onto the street, she was immediately forced to leap backward, pressing herself against the wall of a building, as a man driving a carriage came barreling down the narrow street, holding a whip, scattering the people in front of him. She had a quick view of his face, pockmarked and cruel, and the animal yoked to the carriage—like a huge housecat with an elongated snout—and then the carriage was gone, rounding a dizzying curve in the street, and the crowd flowed back into the space left in its wake.

Dea didn’t move. She closed her eyes and took several deep breaths, trying to stop the crazy pounding of her heart. ‎But every time she opened her eyes, she was gripped by a terrible feeling of vertigo. Or maybe the reverse of vertigo: she felt she was standing at the very bottom of an enormous canyon, and was in danger not of falling down but of being crushed from above.

Because the city she’d entered was built not just out, but up. She must have been on one of its lowest levels, although when she finally managed to take a staggering step forward, she passed over a grate through which she could see people moving below her, their necks yoked with heavy gold bands like the animal pulling the carriage had worn. Slaves.

The street was teeming with people and lined with shops and market stands, and at intervals staircases led up to another level of buildings, where even more staircases wound to another level, and another, and another—so much construction, all of it built on top of previous construction, it seemed impossible the whole city could remain standing.

High above her, so high she had to crane her neck all the way to see it, was the sky, and small dark shapes zipping through it. Birds? She couldn’t tell. Looming above the rest of the city was a tower that seemed to have been hacked out of an ancient mountain, built half of rock and half of spiraling structures of glass—this, too, a construction so absurd and disproportionate it could have existed in no real place.

“Entschuldigung,” a man said, shoving by her. She thought the word was German; all around her people were shouting in various languages, some of which she recognized, some of which she didn’t. A woman was arguing in rapid French with a vendor of mirrors—thousands of mirrors, glinting in the sun, were laid out along a table. Dea had studied French since eighth grade but understood nothing but remboursement, refund. As she passed, she noticed that all the mirrors reflected not the landscape in front of them, but random scenes, maybe other portions of the city.

Or maybe, she thought with a tiny shiver, portions of her world. She remembered her mom had once told her: the mirrors are how they see out.

It was so crowded she had no choice but to move along in the direction of the rest of the foot traffic. ‎It was hot and noisy and it smelled bad, but she didn’t mind the clamor or the closeness: she felt secure here, lost in the crowd, unobserved and unobservable. And she was so stunned by the sheer size of the city and the great canyon walls of its growth that she even forgot to worry about her mother; she just walked, and watched, filled with wonder and awe.

Women hung from the windows of nearby buildings, calling down to passersby in the street. Kids with bronzed arms threw olive pits into the crowd and then ducked away, giggling. Dea saw stacked cages filled with strange hybrid animals: feathered reptiles, horses no bigger than housecats, duck-like creatures with the scales of a snake. She passed a store selling masks that moved and grimaced, like human flesh, and one in which pale white statues chanted and muttered through porcelain lips. Suddenly the street opened up and deposited her in a square: on a cracked stone platform at its center, a man with two heads, arranged not side by side but front to back, was auctioning off slaves, drumming up bids from the assembled crowd. Dea watched a girl, probably no more than eight, her posture distorted by the weight of the two thick bands around her neck, be led to the block, and felt suddenly sick.

The crowd surged forward, flowing down toward the slave auction, but Dea turned and fought her way against the flow of people, reaching a stairway stitched vertically between buildings. She grabbed hold of the railings like a drowning person finding a lifeline, and began to climb.

The staircase corkscrewed several times and then dumped her on another street, this one a good twenty feet above the square, and the shouting, jeering crowd, and the slave auction. It was only slightly less crowded than the street she’d come from. She could still hear shouting from the auction below, and it made not just her ears but her whole body hurt. It didn’t take her long to find another staircase, this one so steep it was practically vertical, sandwiched between a shop that sold nothing but wooden puppets and a tavern from which an alcoholic stink rolled out in waves. She took the stairs, sweat moving down her neck freely now, but also dimly aware that she felt stronger here, healthier than she’d ever been.

The architecture changed as she reached the city’s higher levels, as did the nature of the shops and the look of the people strolling the streets. In the city’s lower levels, the buildings were built primarily of thick blocks of grimy stone, green with age and jammed together like a series of overlapping teeth. But up here, Dea saw steel and glass and other modern materials. The streets were quieter, and lined with trees and flowerbeds, and the shops sold sparkling jewelry on velvet cases, including dazzling cat-shaped brooches that paraded back and forth in the windows, flicking their diamond tails; or long dresses made of white silk that looked like bits of cloud. Stone bridges spanned the vast chasms down to the bottom of the city, and even the staircases between city levels were better built, out of winding iron or dramatic marble, with ornately scrolled banisters.

She’d climbed six street levels already; still, she was only about halfway to the final level, and the tower that twisted like a ribbon of steel and stone toward the sky. But at the top of the next stairs, two men were standing, blocking her passage. Dea drew back, swallowing a sharp cry as they turned to face her: each of them had only a single eye, large and unblinking, and a mouth full of hundreds of sharp, overlapping teeth.

For a second she just stood there, frozen, fighting the urge to turn and run.

“Passage interdit,” one of them said at last. “Niveaux privés.”

“What?” It was so shocking to hear the thing—the man—speak well-accented French, Dea temporarily forgot to be afraid.

He switched seamlessly to English. “Restricted access,” he said. “You have a pass?”

Dea shook her head. “I—I didn’t know I needed one.”

The men kept on staring. They couldn’t help it, Dea supposed, but it made her stomach turn: their eyes were enormous, like the lenses of a telescope. “Levels nine and above are restricted access,” one of them repeated. Dea wondered whether he was being deliberately unhelpful, or whether he simply didn’t know very many English phrases.