During the fourth and fifth centuries after Christ, the ordered landscape of the Roman world suffered a process of transformation, no doubt gradual in its development but, as regards its ultimate effects, certainly disastrous. The imperial system was slowly breaking up; and, while the great landowners withdrew to remote fortified demesnes (where, if they were originally of Roman descent, they soon took on the manners and costume of outlandish barbarian neighbours), the huge open cities, which had expanded under the sun of pax romana, with their libraries and their baths, their market places and their temples, shrank into smaller and meaner compass, behind massive walls often constructed from the debris of demolished shrines and palaces. Aqueducts had been breached, flooding the farm-lands: as travel grew more dangerous, the post-roads were neglected. Fugitives thronged into the safer townships: the mediaeval city began to appear, picturesque, squalid and overcrowded, with its girdle of crenellated ramparts, its narrow, tortuous streets, its confusion and its poverty.<br><br>For more than a thousand years, almost up to the dawn of the Industrial Revolution, most European cities belonged to the Middle Ages, both in their design and in their outlook. Many of them preserved their gates and walls; and through the gates a citizen could walk without hindrance into the unpolluted countryside. As late as the opening of the nineteenth century, Londoners, though they might grumble at the stink and congestion and noise of their immense metropolis, were never far separated from country sights and sounds. Three windmills could be viewed from the Strand; and even the most sedentary inhabitant of the thoroughfares between Oxford Street and Piccadilly had only to stroll west beyond Hyde Park Corner, or northwards through the fields behind Portland Place, to lose himself in some rambling lane among meadows and market-gardens.