Xander was about fifty, with a round, red face and a shock of greying hair. In his ill-fitting toga and with his fingers coated in sawdust from his toy forts, he seemed remarkably unworldly. But Severa listened to him, and Brigonius began to see that the point of view of a Greek, of a race seen as a cultural ancestor of the Romans, was quite different from that of a subjugated people like his own.

Now Xander smiled. ‘Like Alexander, the emperors have looked beyond the edge of the world and recoiled, their minds fogged.’

Hadrian, a new emperor forty years old, had formulated a new policy. He was intent on abandoning Trajan’s expensively won territories and drawing the empire back into a shell bounded where possible by ‘natural frontiers’: deserts, rivers. And where necessary such frontiers would be fortified.

As Brigonius thought this over the more it seemed a remarkable ambition, for to succeed Hadrian was going to have to reshape the Roman spirit. No longer expansive conquerors, the Romans would have to turn their empire into a community. Even the army would have to be redesigned, turned from a mobile fighting force to one able to defend fixed frontiers.

‘Of course he has his opponents,’ Xander muttered. ‘There are plenty of generals who would like their turn at the booty-gathering of the past. Critics in the Senate say that to sit behind a wall and jab at your unconquered enemies with a spear isn’t the Roman way - but half of them are fools who still dream of the Republic, and scarcely matter. There are some who wonder if, by halting the expansion that has always fuelled Rome’s economy, Hadrian isn’t committing some drastic, long-term error - although no doubt we’ll all be dead before anybody can answer that.’

Karus said, ‘Admirably summarised, architect. But as far as we’re concerned, Hadrian’s dilemma boils down to this. If you’re not going to expand then you have to stabilise the frontiers.’

With typical Roman magnificence, Hadrian intended to consolidate his northern provinces’ frontiers into a single defensive boundary, a system of forts, walls, banks and ditches that would span Europe, heading west from Asia Minor, following the great continent-draining rivers to pass north of Dacia, Noricum and Lower Germany - and then it would leap across the Ocean to Britain.

‘I think it’s clear enough where the British boundary is to be,’ Karus said. ‘To the north of here, in the place where the neck of the country narrows, between the estuaries of the rivers Tinea and Ituna.’

‘Very well,’ said Severa. ‘The question is what will he build there? We can show him all the models and plans we like. But what will convince this man that he must build in stone rather than turf as he did in Germany?’

Karus said, ‘The emperors have been building linear defences of one sort or another since the time of Domitian.’