Today the world is looking again towards high-rise living to deal with housing its burgeoning population. The Romans got there first, writes Prof Andrew Wallace-Hadrill.
When London's population reached the million mark in the early 19th Century, it was the first city in Europe to do so since the collapse of Rome in the 5th Century.
London enjoyed (or soon developed) many of the advantages of the industrial revolution to support it - the steam engine and the railway system, coal and gas power, Portland cement for construction, and so on.
To cope with a rapidly growing population, London spread over the countryside, and at the centre built upwards, though it was not until the 20th Century that the high-rise blocks so typical of the modern city were built. To control the population, including growing numbers of poor and immigrants, Sir Robert Peel invented the Metropolitan Police force.
Rome in the imperial period, for 400 year from the mid-1st Century BC, had to cope with a similar population, though without most of the advantages of modern technology.
Rome had no railways, but it was at the hub of a potent network of communications. The roads radiating from the city received a major upgrade from the first emperor, Augustus, who incidentally established a postal system (at least for government communications) that spread across the empire.
Supplies came in by road, but more importantly by water, down the Tiber from central Italy, supplying bricks for construction and a plethora of other items, and upstream from the ports at the mouth of the Tiber.
Portus, the great artificial harbour built by the emperors Claudius and Trajan, was a major engineering feat, and allowed a steady supply of foodstuffs, grain from Egypt and Africa, oil from Spain, luxury goods including pepper and spices from India.
A substantial imperial bureaucracy organised the supply, and then oversaw distribution within the city, with full citizens benefiting from a monthly hand-out - by the 3rd Century, the hand-out included a generous ration of pork.
As important as food was fresh water, the crucial defence against disease in a crowded city. Rome developed aqueduct technology, bringing water in for up to 100 miles away from the limestone mountains of the Apennines. The arches of the aqueducts, running by no coincidence parallel to the modern railway lines, still impress visitors, though no less impressive but hidden from view are the sections of the channels that run deep under the mountains. Some of these still serve the modern water supply of Rome.
Housing was a major challenge, and to meet it the Romans learnt to build higher and stronger than anyone before.
The use of concrete, based on lime and volcanic sand, permitted the Romans to create new architectural forms, like the dome.
Speed and reliability of construction was made possible by the use of standardised material for the facing - first in a network of regular blocks of volcanic tuff, then in bricks.
As Rome built higher, brick production around the city boomed. The fact that the producers frequently stamped the bricks with their names helps us to date such construction closely.
Such multi-storey blocks, typically with shops on the ground floor, and rental apartments on two or more floors above, were called insulae or "islands" - they might occupy an entire city block, with roads flowing around like the sea, though sometimes a city block could contain several insulae.
They were typical of the metropolis. Outside Rome, they are rare, and Pompeii has no examples, although the one example from Herculaneum, built in the first half of the 1st Century, shows the sophistication of construction, with latrines and drainpipes even in upper apartments linking down to a great sewer below ground.
The port city of Ostia has numerous examples of such brick-and-concrete insulae, preserved to two or three floors, and it is clear that some at least of the apartments could be commodious and well-lit.
But it is Rome itself that preserves the best examples, including the insula at the foot of the Capitoline Hill which survives to five floors, and illustrates how independent units could be packed into the available space.
Insula living was a familiar feature of 1st Century Rome, and the satirists, Martial and Juvenal, complain of the exertions necessary, and the dangers run, if you lived on the top floor. (The penthouse suite only became desirable after the modern invention of the elevator.)
All sorts will have lived in such insulae - while the typical rich person still had an independent houses (domus), most people, the comfortably off as well as the poor, lived in apartments.
Nor was it necessarily just one family per apartment - you might sublet rooms, and the lawyers of the time explained the nightmare of trying to identify who was legally liable, owner, tenant or sub-tenant, when a passer-by was injured by a chamber pot carelessly thrown out of an upper window.
Tens of thousands of such apartment blocks were needed to house Rome's enormous and fluid population - a listing of the buildings of the city puts them at more than 44,000, although if this is right, some must have been rather small.
Certainly, we can see their ubiquitous footprints on the vast map of the city's streets that was incised on marble slabs under the emperor Septimius Severus. Though only fragments survive, it is truly impressive, the first known instance of a detailed map of the streets and buildings of any city.
That fact underlines how important it was to the city authorities to know their city and its inhabitants in detail. It was Julius Caesar who as dictator first surveyed the city, district by district and property by property. It helped the government in many ways, not least to cut down on the list of citizens claiming free hand-outs of food. Owners of properties were responsible for reporting inhabitants to the city prefect.
In these contexts, we meet the figure of the insularius, the caretaker responsible for an insula. He also had to ensure that his property was supplied with fire-fighting equipment, buckets and axes. He was thus the point of intersection between inhabitants and the forces of law and order, and under imperial rule, such forces multiplied.
The military urban cohorts under the city prefect, and the fireguard, the "vigiles" under the fire prefect, had the power to enter premises and check all was in order.
At 7,000-strong, the ancient fireguard outnumbered the modern vigili del fuoco, who see themselves as their descendants. And if they had no fire engines, at least they had force-pumps called "siphons", built to a design of Greek hydraulic engineering.
Modern technologies may be different from those of ancient Rome, but in both cases, a vast population produced massive challenges, which the Romans addressed in ways scarcely less impressive than those today.
More from the Magazine
The British Museum's 2013 show of artefacts from the Roman cities of Pompeii and Herculaneum, buried in ash during an explosive eruption of Mount Vesuvius, was a sell-out. But could even greater treasures - including lost works of classical literature - still lie underground?
Building the Ancient City: Athens and Rome is a two-part series, presented by Prof Andrew Wallace-Hadrill, broadcast on 20 and 27 August on BBC Two at 20:00 BST - or watch on BBC iPlayer
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