On the track the first cohort was fifty paces from the phalanx, coming on steadily, hobnailed boots crunching on the ground and armour rattling and bumping.

‘Come on, boys!’ the centurion called, then his head jerked back as a bolt drove through his iron helmet with such force that the point came out of the back. He fell, tripping a man in the front rank.

‘On, lads, on!’ The cohort’s five signifers were jogging alongside the column, for it was not their job to lead in the last moments of the charge. Today, the aquila remained in camp, but the signa were important, and the men who carried them carefully chosen. ‘Go on!’

Another centurion ran to catch up with the men in the lead. ‘Steady the First! Keep going!’

They were forty paces away. A stone came throbbing through the air, pale white and big as an orange. It came from the left, from a slit in one of the towers, and knocked the crests from the helmets of half a dozen men in the column.

‘Pila!’ the centurion shouted. Legionaries in the first rank raised their heavy javelins ready to throw. The formation was a little ragged as they had doubled down the track, and a man in the second rank cursed as the butt of a pilum knocked his shoulder.

Another stone-throwing engine shot, this time from a tower on the other side. The missile hit the ground, just short, before bouncing up to take the leg off a signifer.

‘Wait for the order!’ the centurion, the princeps posterior of the cohort, was running just beside the front rank now. They were thirty paces away. Arrows flew from the far side and the walls, pattering against shields. No one fell.

Down in the ditch Ferox ducked to dodge an arrow that had come over the line of shields held up by Vindex and his men. ‘Come on, quickly now.’ He pointed up at the bridge. The side wall hid the pikemen from the waist down, but their chests, shoulders and heads were exposed. Ferox had gone to the left, because that meant that he and his men would be on the right of the phalanx, the unshielded side. ‘You!’ He tapped one of the legionaries on the shoulder. The man’s face was pale and one hand stroked the side of his machine. ‘Kill the man in the front rank. You, the one behind and you, start at the back. Then work your way along. Got it?’ They nodded and one grinned.

As the first cohort closed, the leading ranks were shielded from most missiles by the phalanx. The stone throwers switched their aim, and one shot cut a swathe through the middle of the formation, smashing the heads of seven men in turn. Those around were sprayed with blood, brains and fragments of helmet and bone. One bled to death after the ragged edge of a torn-off cheek piece slashed into his neck.

‘Front rank!’ the centurion called. The six men lobbed their pila, arcing them up because the range was still fairly long. There was a rattling as most hit the shafts of the pikes held up by the men in the rear of the phalanx. One slipped through and struck the forehead of a pikeman, just below the brim of his helmet. He staggered, then fell forward, and the man behind stepped onto his body to take his place.