As the riders approached the camp, they reached the first group of soldiers exercising at the palus, a wooden stake the size of a man. The legionaries stood crouched before their targets and alternated between thrusting their swords at the posts, and smashing their shields into them. Marcus was familiar with the technique from his days at the gladiator school. The centurion in charge of the soldiers glanced up but did not salute. His new commander was wearing a simple cloak and no sign of the authority granted to him in Rome. Caesar nodded a greeting as they pounded by.

It was different at the gate to the camp, though. There a timber bridge extended across the ditch and a section of fully armed men stood guard on the far side. Caesar reined in and walked his horse across the bridge, its hoofs making hollow thuds. The duty optio held up a hand and stood in his way.

‘Halt! What is your business here?’

Caesar tugged lightly on his reins and reached into the bag hanging from one of his saddle horns. ‘Bear with me a moment, I have it here ... somewhere.’

The optio puffed his cheeks impatiently. ‘If you’re the grain merchants the quartermaster’s been waiting for, then you’re late and I warn you he won’t be a happy man.’

‘No, not grain merchants,’ Caesar mumbled as he continued rummaging. Then he smiled as he withdrew his hand and held up a baton, gold at each end with a strip of parchment tightly fastened round it by the great seal of the Senate and people of Rome. ‘Here we are! I am Caius Julius Caesar, governor of the province of Gaul and general of this army. I am here to take up my command, under the authority of the Senate.’

Marcus saw the optio s eyes widen as his jaw went slack. Recovering quickly, he stepped smartly to the side, stood to attention and snapped his fist across his chest in salute.

‘My apologies, sir.’

‘At ease.’ Caesar laughed. ‘Well, I’ve never been taken for a grain merchant before!’

‘No, sir. Sorry, sir.’ The optio’s face reddened.

‘No need to apologize. We’ve been on the road for five days. Carry on, Optio.’

Caesar urged his mount forward and led his escort into the camp. Beyond the gate Marcus took in a sharp breath as he saw neat lines of tents stretching out in every direction. Smoke drifted up from scores of campfires and the forges of armourers. The air was filled with the sound of voices, and the shout of orders. Ahead of them stretched a long, wide avenue reaching into the heart of the camp. Some of the soldiers looked up curiously as the riders passed by, but most simply ignored them and continued with their duties, or sat outside their tents tending to their kit or playing dice.

When they reached the large tents at the centre of the camp Caesar was halted by a centurion of the elite unit of soldiers entrusted with guarding the headquarters and the senior officers of the army. As soon as he saw the baton he waved the riders through and they dismounted at the horseline outside the largest of the tents. The eagle standards of the four legions stood on a podium in front of the entrance and were guarded by eight men with bearskins covering their helmets and shoulders.