Hadrian had to consider fixing his borders because his empire had reached its natural limits of expansion. In Britain, from Claudius’s first foothold at Rutupiae in the far south-east of the island, a wave of military advance had washed over the countryside, leaving behind a network of forts, roads and signalling towers. Brigantia had been the most northern political unit the Romans had been able to deal with diplomatically. But under the Emperor Domitian governor Agricola had gone further, mounting a strong assault on the furthest north, and his ships had sailed all the way around the northern coast. But in a land of mountains, mist and bogs, inhabited by an elusive, fleeting people who never seemed to understand when they were defeated, it had not been a comfortable campaign for the Romans, and Agricola’s forces, scattered in garrison forts, were overstretched. When troops were withdrawn from Britain to deal with problems on the Danube the Romans had been forced to fall back to Brigantia.

Elsewhere in the empire similar limits were reached. Domitian’s successor Trajan was another expansion-minded soldier-emperor, and he had won swathes of territories in the east of the empire. But towards the end of his life his energies had been consumed by troubles along the empire’s long, vulnerable borders, in Mesopotamia, Africa, even in northern Britain once more. And in the east, while he was distracted, the Jews had risen in a savage revolt, that had been put down equally savagely.

This was the complex, unstable situation bequeathed to Hadrian, Trajan’s successor in turn, who had served with his mentor in the field.

‘You must understand the way these Romans think,’ said architect Xander. ‘Once the Romans saw no limit to their conquest. Why should they? Especially once their generals started getting rich from it. Expansion pays for more expansion, and on it goes. Why shouldn’t it proceed to the ends of the world? But the truth is the Romans can venture no further than the plough ...’

As the Romans had expanded out of their Mediterranean heartland, north and south, east and west, they had taken lands that were already farmed, and which could provide the wealth to make their city rich, and to fuel their further expansion. But Rome could not conquer wastelands. So the empire was like a sea, said Xander, rising to flood the agricultural heart of Europe. But it was dammed in the east by the Parthians, and in the north, west and south its waves broke on shores of forests, deserts and mountains.