Alexandria was not a Roman city. It was a Greek city under Roman administration, founded by Alexander the Great in 331 BC and designed from its inception as a world capital — a city that would connect the Mediterranean world to Egypt and, through Egypt, to the trade routes of the East. By the time Rome absorbed it as part of Egypt following Cleopatra’s death in 30 BC, Alexandria was already three centuries old, the second-largest city in the Mediterranean after Rome itself, and possessed of institutions — the Library, the Museum, the great lighthouse — that Rome had nothing to rival. The Romans did not conquer Alexandria so much as inherit it, and the inheritance was complicated.

Antioch on the Orontes — modern Antakya in southern Turkey near the Syrian border — was the third city of the Roman Empire and the capital of its eastern operations. After Rome and Alexandria, no city in the Mediterranean world was larger or more strategically important. It was the administrative center for the Syrian provinces, the supply base for Rome’s eastern wars, the commercial hub connecting the Mediterranean trade network to the silk and spice routes of Asia, and an early center of Christian organization so significant that the word Christian — Christianoi — was first used there. The city that matters to understanding Rome’s eastern empire is Antioch, and it is among the most underrepresented in the popular historical imagination.

Ephesus was the most important city in the Roman province of Asia — which meant it was one of the most important cities in the world. At its imperial peak in the second century AD, its population may have reached 200,000 to 500,000 people, making it one of the three or four largest urban centers in the Roman Empire after Rome itself and Alexandria. It sat at the western terminus of the major trade routes from the Anatolian interior and the eastern Mediterranean, on a harbor that connected it to the Aegean sea lanes, and it possessed in the Temple of Artemis one of the seven wonders of the ancient world. What the site preserves today — the most extensively excavated and partially reconstructed ancient city in the world — is an archaeological window into Roman urban life of a quality available nowhere else.

The Romans did not found London because they needed a city there. They founded it because they needed a crossing point on the Thames, and the crossing point became a city because trade and administration followed the military logic that had chosen the site. The settlement that grew up at the first substantial tidal ford on the Thames — approximately where London Bridge stands today — was called Londinium, and within a century of its founding it had become the administrative capital of the Roman province of Britannia and one of the most important commercial cities in the northwestern empire. Britain was at the edge of the known world; Londinium was a world city transplanted to the edge.