As for Audax, he could have been executed with no questions being asked at all - or at the minimum tortured, for under Roman law slave testimony was only valid if extracted under torture. But if Tarcho had saved Thalius it was Constantine himself who saved Audax. In those moments when they had been joined in an embrace of life and death, the Emperor had seen something he liked in the slave, and he had pledged to protect him. When the fuss had died down Thalius hastily granted the boy his freedom and gave him into the care of Tarcho, who he judged was likely to do a much better job of keeping the boy safe than Thalius himself ever could.

As for the other principal in the drama, Ulpius Cornelius had made noises about the betrayal of his trust, receded into the shadows of the court, and Thalius had never seen him again. And he never knew if Cornelius had been complicit in the attempted assassination - if Thalius was the only dupe.

Tarcho had made good the Emperor’s promise that Audax would have the chance to try life as a soldier. At the age of sixteen he was enlisted into the frontier garrison at Banna. He immediately flourished under the healthy food, medical supervision and training regime of the army; by the time he was eighteen he had shed the last shadow of the pale-as-a-ghost slave boy Thalius had dug up from the mine.

But he had rapidly proved too effective to be wasted in the stasis of a frontier post. On a letter of recommendation from Banna’s commander, Audax was transferred away to the field army units in Gaul. Thalius saw him only rarely after that.

Audax was too young to fight in Constantine’s first serious engagement with Licinius, Emperor of the east. It was a partial victory for Constantine; Licinius ceded territory but survived. The showdown came ten years after Constantine’s visit to Britain, and by now Audax was old enough to serve.

‘It was magnificent, Thalius,’ he said now. ‘They say it was the largest war for a century - there were perhaps a hundred and fifty thousand men on each side, and it raged across Europe and Asia for a year before Constantine’s final victory near Byzantium ...’

Audax forbore from telling Thalius any war stories, and the older man was glad of it. The civil war had been another terrible internal grinding-up of resources that could surely have been better deployed against external enemies, like the Franks and the Alamanna, new barbarian federations on the Rhine border, and the Goths on the Danube, and the revived Persians in the east. Even while Constantine fought Licinius, Visigoths had taken the chance to cross the Danube, and Constantine found himself at war along a front three hundred miles long.