He brought his knee up hard between the man’s legs and felt the kneecap thud into his genitals. The man gasped and rolled his eyes and for an instant his hands loosened their grip.With a convulsive heave of his whole body Cato thrust him off, and then stabbed his sword into the man’s side as soon as his right arm was free. The blade slid out of the wound with a wet sucking noise and Cato scrambled back on to his feet. On either side his men had cut down several more of the enemy, but already many more were appearing in the glow of the flames. Far too many to take on, and with the confidence of numbers the enemy surged towards the Romans. Cato realised that he and his men had done all that they could. To remain here for another instant was to invite death.

‘Fall back!’ he cried out. ‘Go!’

He turned and raced away from the enemy, between the burning onagers, and back towards the safety of the darkness. His men hurried after him, breathing heavily from their exertions and excitement. The enemy came on, rushing after the Romans in a wave. Some realised what their true priority was and leaped towards the blazing onagers, heedless of the scorching heat as they desperately struggled to pull away the blazing wood piled round the thick timbers of the frames. A few scooped up sand and tried to smother the flames, while others pulled off their cloaks and tried to beat the flames out. But more, many more, were filled with a desire for revenge on those Romans who had dared to venture from the fort to attack their camp.They charged past the burning onagers and rushed after Cato and his men, pursuing them into the darkness beyond the orange loom of the flames.

‘On me!’ Cato called out. He wanted his men close, to make sure that they passed through the defences together. To his right was the dark bulk of the fort, with torches flaring in each of the corner towers. And there, halfway along the wall, the spark of light from the oil lamp, and behind, at an angle, the dimmer flame of the lamp in the window of the headquarters building.

‘Keep going,’ Cato muttered to the dim shadows beside and behind him. Further off he heard the shouts of the men pursuing them. ‘Stay with me.’

They ran on, instinctively edging towards the fort as the two small lights closed on one another. Then the inevitable happened. Just as Cato reached the point where the flames overlapped there was a cry of pain just behind him. He spun round and saw a dark shape rolling on the ground, groaning through gritted teeth.

‘What’s happened?’

‘It’s Petronius, sir. He’s stepped on a caltrop.’