‘Check inside that building. Make sure everyone’s all right. Get any casualties seen to by the medical orderlies. Move!’

As soon as he had given the order, Macro re-joined Cato. The first of the onagers was almost ready to load and in the growing light they could see two men struggling to lift a rock into the cup at the end of the throwing arm. An order was shouted and an instant later the beam of wood shot up again, cracked against the cross bar and another missile arced towards the fort. As before, it seemed to be coming straight for them and Cato glanced at Macro. Macro was tracking the rapid progress of the rock, so Cato forced himself to remain composed and resist the impulse to dive to one side.The rock struck the base of the gatehouse and Cato felt the shock of the impact right through his body. A chunk of masonry fell off the rampart close by and dust and grit tumbled down from the dry thatch roof overhead.

Macro looked at him. ‘You all right?’

Cato nodded.

‘Better check the damage.’

They leaned over the rampart and gazed down. The rock was still in one piece where it had bounced back from the wall and there was a small crater on the face of the masonry, near the arch, and a skein of small cracks radiating from it.

Macro winced. ‘I really hope that was a lucky shot.’

The second onager swung into action and another stone flew through the air towards the fort. Again it fell short and bounced off the stony ground before harmlessly hitting the base of the wall beside the gatehouse. As dawn broke over the desert the bombardment continued in a steady rhythm of the clanking ratchet, the crack of the throwing arm striking the cross beam and the crash of the impact. But nearly half the shots fell short, or went wide and hit the walls, or overshot the defences and smashed into the buildings beyond. Every hit on the gatehouse dislodged more masonry and the fine cracks gradually widened. One lucky shot landed right on the bottom of the gate itself, making the hinges rattle. Macro gave the order for most of the men to shelter behind the wall, leaving those manning the corner towers to keep an eye on the enemy. After a little while Macro and Cato climbed down from the gatehouse and sat down in the watchroom beside the timbers of the gate.

‘Have you ever been on the receiving end before?’ Cato asked.

‘No. Can’t say I’m enjoying the experience.’ Macro smiled faintly.’Have to hand it to Bannus and his Parthian friends - they’ve managed to spring a very nasty surprise on us. And I let those bloody onagers slip by, right under my nose.’

‘Don’t be hard on yourself, sir. No one could have seen that one coming.’