It is the fourth year of the Genroku era, 1701, in the eastern capital. The cherry blossoms have fallen along the Sumida River, and the first summer fires are being lit in the watchtowers of the daimyo residences. A city of more than a million souls stretches between the shogun's castle and the flatlands of the east, larger than London, larger than any city in Europe, yet unknown to the world beyond its shores.

The limestone plateau rises from the western desert, eight kilometres southwest of the city that Egyptians will one day call Giza. In the season of Akhet, when the Nile spreads across its valley, twenty thousand workers gather where nothing stood two decades before. They have come to finish a mountain built by human hands, its white casing already catching the dawn light as the final block is drawn toward the summit.

It is the year 1487. The great temple of Huēy Teōcalli has just been rededicated, and the city of Tenochtitlan stretches across its island in Lake Texcoco, connected to the mainland by three great causeways of stone. Two hundred thousand people move through its canals and markets, in a metropolis larger than any in Europe at this hour.

It is the year 780 of the Common Era. The muezzin has not yet called the fajr prayer, but lamps already flicker in the paper workshops beneath the citadel of Afrasiab. A merchant from Chang'an, having crossed the Pamirs with his pack animals, sleeps now in the courtyard of a caravanserai near the Oxus road. His bales of raw silk wait in the warehouse. By noon, Sogdian brokers will weigh them against silver dirhams minted in Baghdad. This is Samarkand, where the Tang frontier meets the Abbasid caliphate, where lapis from Badakhshan changes hands for porcelain from Xi'an, and where the old painted halls of Sogdian nobles still stand, their frescoes dim in the lamplight, even as new mosques rise in the suburbs beyond the walls.

You stand on the eastern bank of the Trave River, where the water widens into a sheltered bay before meeting the Baltic Sea. It is the year 1143. Count Adolf II of Holstein has chosen this spot, a day's sail from Denmark and within reach of Slavic territories to the east, to plant a new settlement. The ground is soft. The air smells of tidal mud and pine resin from the surrounding forests. Somewhere beyond the tree line, a smith hammers out the first iron nails that will hold together the wooden palisades of a town that does not yet know its own future.

It is April of the year 1100. In the harbour of Genoa, forty galleys ride at anchor beneath the steep limestone hills. Sailors unload bales of Syrian cotton and jars of Palestinian wine, the first fruits of a treaty struck with Bohemond of Antioch during the recent crusade. The city is barely two miles long, pressed between the mountains and the Ligurian Sea, yet its merchants have already begun to build something that will outlast any cathedral wall. You are in the harbour of Genoa, at the beginning of three centuries that will carry these ships to the Black Sea, to the alum mines of Phocaea, and eventually to the fatal waters off Chioggia.

It is a morning in June, in the year 79 of the common era. The sun rises over the Bay of Naples, and in the city of Pompeii, a woman steps into her atrium to hear water dripping from the compluvium onto the impluvium below. The mountain Vesuvius stands to the north, as it has for every morning of her life, its slopes green with vines and silent.

It is the autumn of 1520, and the body of Sultan Selim I has barely reached Istanbul when his only surviving son, Suleiman, twenty-six years old, enters the city through the ancient walls to claim the throne. The capital he inherits already holds perhaps four hundred thousand souls, making it the largest city in Europe, larger than Paris, larger than Venice at the height of her power. From the Arsenal on the Golden Horn to the Asian shore at Üsküdar, from the dried-cypress grounds of the old Byzantine hippodrome to the new palace rising on Seraglio Point, this is the city Constantine built, Mehmed conquered, and which now, under Suleiman, will become the seat of the most powerful empire the Mediterranean world has known.

It is the year 900, in the early months of spring. A clinker-built longship lies ready at the head of a deep Norwegian fjord, its striped sail furled against the mast, its crew of thirty men preparing to row out on a long crossing of the northern seas. The voyage ahead will last many days, through waters they know only by the memory of those who have returned.

You are standing on the bank of the Eurotas river, where the water runs clear and cold from the Taygetus mountains. It is the year 450 BCE. The morning sun has risen over a city that has never known defensive walls, where low houses of sun-baked brick cluster along the river plain, and the smoke of breakfast fires drifts thin across the olive groves. Men are already walking toward their common mess halls, carrying the prescribed contribution of barley meal and wine.