What makes a city? A simple mass of people, a great temple, a hub of learning, trade or transport? Colin McEvedy's idiosyncratic book, a survey of 120 "centres of ancient civilisation", doesn't ask the question, but unwittingly suggests some answers. In doing so, it tells us almost as much about contemporary urban life as it does about the distant past.

An incomplete catalogue of every city of significance in the Roman and Mesopotamian worlds, it takes in backwaters and glittering capitals from Lincoln to Lyons to Alexandria. The author was an intriguing character who, as well as being a psychiatrist at Ealing hospital, was a historian, demographer and armchair archaeologist, and had yet to finish his magnum opus when he died in 2005. This book represents the efforts of his family and editors to collect what had already been written  and sketched out, for the mapmaking was an important part of the project  and present it in something like the form he intended.

The result is a genially written encyclopedia, with each entry telling us something of the history, topography and size of the city concerned. McEvedy's particular obsessions are threaded through the book  he jousts with other scholars over population estimates  but he has a light enough touch that the prose is always readable. The maps that accompany the entries are all done to the same scale, making it possible to compare, say, the vastness of Alexandria, whose walls enclosed an area of more than 1,000 hectares, to puny, 65-hectare Pompeii. Though well-executed, they're a bit too reminiscent of diagrams in geography textbooks, passing up the opportunity for something more gorgeous.

The stories of the rise and fall of each city are dealt with matter-of-factly. Some clung on to rise again in modern times, others disappeared practically without a trace. You can almost hear the sands sweeping over deserted forums as McEvedy points to a lack of adequate defences or silted-up harbours before moving on to the next. The main reason most of these settlements failed was the decline in prosperity and effective political organisation as the Romans retreated. This did for most of the northern towns they created or made flourish. In the east, dynastic squabbles and changes in patronage were as often the agents of urban decay. Memphis gave way to Alexandria as the Greeks stamped their rule on Egypt; Babylon to Seleucia on the Tigris under similar circumstances. Seleucia in turn was abandoned by the Parthians in favour of Ctesiphon.

At the time, Paris and London, even Jerusalem, were poor and ramshackle. Carthage, Ephesus and Antioch were the jewels of the age, drawing to them peoples from across the Mediterranean, enriching themselves and building great public works. They didn't, of course, make it through the sieve of history (though modern Antioch, Antakya, is a medium-sized Turkish city). Those that did survive have a very different face today: most of McEvedy's cities had a population of around 10,000. Ancient Rome and Alexandria may have reached 250,000, but that is only the size of modern Wolverhampton. In the 21st century, London's population laps up against the eight million mark, which is still ten million less than Shanghai.

Is there any meaningful sense in which these seas of people are "cities"? What is the relationship between Seleucia and Los Angeles? Ironically, given his fixation with it, what McEvedy shows us is that it's a mistake to get too hung up on population. The places he deals with were tiny by modern standards, but loom large in the imagination, suggesting that the idea of a city may be more important than its physical properties. And beyond a certain minimum level of civic pride expressed in stone or steel or glass, the essence of a city lies in its relationship to a hinterland and to other cities. Shanghai could well be to a Chinese villager what Rome was to an Etruscan farmer: a centre of wealth, power and spectacle. And Shanghai will vie with Tokyo and São Paulo for trophies such as the Olympics, the headquarters of international institutions and other wonders, just as its ancient counterparts did. There's another lesson, of course, in McEvedy's work: cities have a lifespan. Only a lucky few pass through that historical sieve. In the ancient world, they thrived for perhaps 1,000 years before falling into ruin. How long will ours last?