Longinus considered the prospect Cato had envisaged for him, and then looked at the young officer with a cool smile. ‘You have a devious mind, Centurion Cato. I would hate to have you as a political opponent. Worse still, as one of Narcissus’ lieutenants. Then you would truly be a man to be wary of.’

‘I’m a soldier, sir,’ Cato replied stiffly. ‘That’s all.’

‘That’s what you say, but this document gives the lie to that.There’s far more to you and Macro than meets the eye. But no matter.’ Longinus tapped his fingers on the desk for a moment and then nodded. ‘Very well, let’s do as you suggest. But there’s one thing that still puzzles me.’

‘Sir?’

‘I’ll accept that the Parthians could have got some intelligence about Bannus, but how could they come to know of Narcissus’ suspicions about me? They would have to have spies right in the heart of the imperial service. That, or spies on my staff . . .’ A brief startled look flashed across the Governor’s face, but before he could continue there was a strident blaring from a trumpet, the notes blasting out across the fort from the direction of the west-facing gatehouse.

Longinus looked at Macro. ‘What’s that?’

‘The alarm signal, sir.’ Macro turned to Cato.’We have to go.’

‘Wait!’ Longinus rose from behind the desk. ‘I’m coming too. And you, Postumus.’

Outside, men were still tumbling from the barracks, clutching equipment as they hurried to take up their positions along the walls of the fort. They stood aside to let the officers trot past and Macro and the others reached the watchtower sweating and breathing heavily. On both sides the auxiliary troops were forming up in sections, sun glinting off their polished helmets as they fastened or adjusted the last items of equipment and then raised their shields and waited for orders. Several sections had been armed with compound bows and they were hurriedly stringing them, one end braced against a boot as the men strained to bend back the other end and attach the loop of the bowstring. The officers in the tower lined the parapet and stared out along the track to where, some distance off, a handful of mounted men were galloping towards the fort. Behind them raced a much larger force.

‘Who the hell are they?’ asked Longinus.

The two parties of horsemen were still too far away to be certain but as they approached the fort Cato strained his eyes and made out enough details to recognise them for what they were.

‘It’s one of our patrols.’ He turned away, hurried across the tower and called down to the section of soldiers at the gate. ‘Open up! Those are our men in front.’