‘I have arranged a discreet meeting,’ Volusianus said. ‘The officer should be here soon. You will remain. As you know, in a few days I will leave with the imperial entourage for the north. The Emperor Gallienus is determined to cross the Alps and crush Postumus the pretender, who holds Gaul. Should anything happen to me, I want a trustworthy man to know what has been arranged.’

Murena acknowledged the compliment with a nod.

Volusianus said no more. He sat waiting, perfectly at ease in the silence.

As the leader of the frumentarii, Murena reported to the Praetorian Prefect. Volusianus answered directly to the Emperor. Apart from the sacred majesty of the Emperor himself, no one was more powerful than the Praetorian Prefect. There was a ritual when a new prefect was appointed. The Emperor would take the prefect’s sword. If I reign well, use this on my behalf. If I rule badly, turn it on me. The prefect commanded the Emperor’s bodyguard. The Praetorians, the twelve thousand most highly paid troops in the empire, guarded the palace in Rome, and accompanied the Emperor on all his travels. Over the years the prefects had also acquired rights of civil jurisdiction, often presiding over court cases in place of the Emperor.

Such power was an evident threat to the Emperors themselves. To counterbalance the Praetorians there were other military units stationed around the capital: the Urban Cohorts in Rome itself, the Second Parthian Legion in the Alban Hills outside. Most Emperors maintained a personal bodyguard of German warriors recruited beyond the frontiers. To watch for disloyalty in each other, usually there were two Praetorian Prefects: Volusianus’s colleague was currently in Milan gathering forces to cross the Alps into Gaul. To curtail their ambition, the men appointed prefect tended to be drawn not from among the senators, the highest social order, but from the equestrians, the second rank. The idea was that the senate would be reluctant to accept a man of lower status than themselves if a prefect murdered his ruler and aimed to take the throne for himself.

None of the carefully constructed checks and balances worked. Only one Emperor had been killed by the barbarians, but several had met their end at the swords of the Praetorians. Two Praetorian Prefects had arranged the assassination of Emperors they had sworn to protect, and assumed the purple in their places. The danger remained. As the poet Juvenal had rightly asked: Quis custodiet ipsos custodies? Who guards the guards?

A tap on the door announced the arrival of the wine. Murena got up, dismissed the soldier, and served the drinks himself.

Volusianus took a deep swig, then warmed his hands around the cup. ‘When I was young – serving on the Danube, the Rhine – I never felt the cold.’

Murena made a vague noise intended both to express sympathy and reject the implication that his superior was old.