It is January 532, in the imperial box above the Hippodrome of Constantinople. Below you, thirty thousand spectators chant the name of their favourite chariot colour, green against blue, as the emperor Justinian watches with his empress. Theodora at his side was born to a bear-keeper and a dancer, raised among the factions whose cheers now shake the marble seats. Within days, these same voices will call for the emperor's head, and the city will burn.

The Tigris River winds through the flat plain of Mesopotamia, its waters brown with summer silt. On the eastern bank, workmen from every province of the new Abbasid empire are tracing circles in the dust with rope and peg. Caliph al-Mansur has chosen this spot, where Persian engineers and Arab surveyors converge, to build a capital that will eclipse every city on earth. Within five years, the walls of Madinat as-Salam—the City of Peace—will rise from nothing.

In the ducal palace at Poitiers, on the first day of April in the year 1204, Eleanor of Aquitaine lies in her final illness. She has outlived two husbands, most of her children, and nearly every rival who ever stood against her. For eight decades she has governed the largest duchy in France, been queen of France and then of England, led armies across continents, and spent fifteen years imprisoned by her own husband for supporting her sons' rebellion. The candles burn low in the chamber where she breathes her last, still Duchess of Aquitaine in her own right, still signing documents that will shape the futures of kingdoms she will not live to see.

In the spring of 1193, in a modest stone chamber in the Citadel of Damascus, Salah ad-Din Yusuf ibn Ayyub lay dying. He was fifty-five or fifty-six years old. Around him stood physicians, family members, and the few possessions he still owned. Outside, the city he had made his capital prepared for mourning. The man who had recaptured Jerusalem, who had faced Richard the Lionheart across a battlefield, had spent his personal fortune on war and charity until almost nothing remained.

In the harbour of Constantinople, beneath the walls of the imperial capital, Egyptian grain ships have begun to dock. It is the year 542, and the Emperor Justinian has reigned for fifteen years. The city holds perhaps half a million souls, the greatest metropolis in the Mediterranean world.

You are standing on the limestone cliffs above the Adriatic, where the city of Ragusa clings to a rocky promontory jutting into the sea. It is the year 1358. The last Venetian governor has just departed through the Pile Gate, and the nobles of the city have gathered in the Rector's Palace to swear an oath of collective rule. For the next four hundred and fifty years, this tiny walled republic between the Ottoman Empire and the Venetian lagoon will preserve its independence through nothing more than diplomacy, trade, and the careful rotation of power among its merchant families.

It is September 1434. Cosimo de' Medici, sixty years old, richest citizen of Florence, rides through the Porta al Prato after a year of exile in Venice and Padua. The bells of San Giovanni are ringing. He does not enter as a conqueror. He walks home to the Via Larga, past the unfinished cathedral with its gaping hole where the dome should rise, past workshops where bronze founders stoke their furnaces and painters grind lapis lazuli to powder. The republic still stands. The Signoria still meets. Yet everything has shifted.

In the year 1253, a Franciscan friar named William of Rubruck reached a city of stone walls and felt tents, built on the open grassland where no city had ever stood before. He had travelled five thousand miles from the court of King Louis IX of France to reach Karakorum, the capital of the most powerful empire on earth.

It is the year 1450 before the common era, in the reign of Thutmose III. The Nile has begun to swell at Elephantine, its waters brown with silt carried from the mountains of Ethiopia. In the village of Deir el-Medina, on the west bank at Thebes, a tomb painter named Kha wakes before dawn to walk the path to the Valley of the Kings, where he will work by lamplight on the burial chamber of a pharaoh who has not yet died.

It is the Ides of March, and the dictator Gaius Julius Caesar lies bleeding on the floor of the theatre of Pompey. The republic he had dominated convulses in shock. In his will, to the astonishment of the assassins, Caesar has named a great-nephew: Gaius Octavius, a sickly eighteen-year-old studying in Apollonia across the Adriatic. The boy will cross stormy seas to claim his inheritance. No one yet suspects what he will become.