The confident atmosphere that had prevailed all morning evaporated. What had seemed like a cushy number was going to be the death of them all. No one spoke as they watched the enemy moving uphill, taking their time to conserve their horses' energy. Having fought the Romans before, Mithridates' men would know that they were at no risk from javelins until they were within thirty paces, perhaps fifty down an incline like this. The ballistae were still within the walls, so there was no means of preventing the enemy from ascending the slope unchallenged. The Pontic horse would have ample time to regroup before charging. Romulus' mouth felt dry at the prospect.

An uneasy silence reigned over the Twenty-Eighth; angry shouts and cries rose from the camp as the rest of the army struggled to get ready. Six centuries of roughly eighty men had to join up to form a cohort; ten of these assembled units made a legion. While the process happened smoothly, it took time. A good general did not march his men out to battle unprepared, thought Romulus. He and his comrades would just have to manage.

It was not long before the enemy host had come to within two hundred paces of their position. Now Romulus could make out the slingers and the archers. Clad in simple wool tunics, they were similar to the mercenaries he had fought against in Egypt. Each man carried two slings, one for short range and another for longer distances. The spare was wrapped around their necks while a leather pouch on a strap contained their ammunition. Many also carried knives. Dressed in white tunics, the archers were better armed. As well as their recurved bows, many wore swords on their red leather belts. With occasional hide or linen cuirasses and helmets, these were troops which could close with the enemy as well as fire arrows from a distance.

Yet neither type would pose a threat to the legionaries' shield wall, Romulus thought. It was the men in the chariots behind, and the heavily armed horsemen on either side, who would do that. Although he knew of the Persians' disastrous attempt to use scythed chariots against Alexander at Gaugamela, Romulus still felt uneasy. The men around him had not been shown how to fight such vehicles, as Alexander's had. Pulled by four armoured horses and controlled by a single warrior, they had curved blades as long as a man's arm protruding from the end of the traces and from both wheels. They promised devastation.

Nor had the Persian chariots been backed up by heavy cavalry, as the Pontic ones were. These horsemen could sweep around to their rear and thus prevent any retreat. Dread surged through Romulus at the memory of the Parthian cataphracts. With conical iron helmets, scale mail which reached below the knee, and carrying long javelins, those opposite closely resembled the mailed warriors who had smashed apart Crassus' legions with such impunity. The sun's rays flashed off the chain mail covering their horses' chests and flanks, reflecting blinding light into the legionaries' faces.

The threat posed by Pharnaces' army was sinking in around Romulus. Men were looking very uneasy. If they knew what I had seen at Carrhae, he thought, many would run now. Thankfully they didn't, so their wavering lines held. Their optio looked to the centurion, who cleared his throat self-consciously. 'Steady, lads,' he ordered. 'We won't have to hold the bastards for long. Caesar is on his way.'

'Fucking well better be,' commented Petronius.

Nervous laughter rippled through the ranks.

They had little opportunity for any further contemplation as the Pontic archers and slingers loosed their first volley. Hundreds of arrows and stones shot up, darkening the sky. This was the opening gambit of most battles, aimed at causing maximum casualties and softening up the enemy before a charge. Although his shield was made of layers of hardened wood and covered with leather, Romulus still felt his jaw clench.

'Front rank, on your knees!' shouted the officers. 'The rest of you, shields up!'

Hundreds of scuta banged off each other as men rushed to protect themselves. Those at the very front, including Romulus and Petronius, did not do the same. Instead they dropped to the ground, allowing their shields to cover them completely, while the men in the second row angled theirs obliquely before them. Those further to the rear held their scuta directly over their heads. This was a method used by the Forgotten Legion to withstand Parthian arrows, and Romulus was pleased to note that Caesar used it too. The normal deployment – with the front row remaining on their feet – allowed many soldiers to suffer injuries to their lower legs from well-aimed shafts.

There was a heartbeat's delay, and then the air filled with gentle whirring sounds as the arrows came down to earth. An instant later, loud crashes announced the stones' arrival too. His muscles tight with tension, Romulus waited, knowing what the next noise would be. He hated it as much as the first time he had heard it. Listening to men scream was much harder to do now than during the rage and immediacy of one-on-one combat, when it became part of the red-hot blur of battle.

Sure enough, strangled cries of pain broke out everywhere. Soldiers collapsed, thrashing at the shafts which had found the gap between shields to pierce their flesh. Others had gained enough momentum to drive through the legionaries' scuta and into their arms and faces. Fortunately, most of the stones just clattered off the shields and bounced away, but a few did find targets, cracking bones and denting helmets. Given the number of missiles released, it was inevitable that there were fatalities. Not many, but the unlucky few slumped to the dirt, their weapons falling from slack hands.

Romulus' dream of getting to Rome was fading. He gazed uneasily at the massed enemy ranks, asking for Mithras' continued favour.

Everyone else was praying to their favourite gods too.

Their work done, the slingers and archers fell back. It was time for the chariots to attack. Romulus could make out at least fifty. Enough to hit most of the Twenty-Eighth head on, while the Thracians and Pontic heavy cavalry rode around to their undefended rear. Their situation was grim now, even critical. Still there was no sign of Caesar or the other legions.

Flicking their reins, the charioteers encouraged their horses into a trot. At last it was possible to make them out clearly. Clad in composite scale cuirasses and laminated armpieces, their crested Attic helmets were not dissimilar to those worn by junior Roman officers. Each carried a long-handled whip, which he used to encourage his mounts to the trot. A moment later, it was the canter. Having conserved their steeds' energy, they had room to ask everything of them. With jingling traces and the blades on their wheels spinning and flashing, the chariots surged forward. Although the slope was steep, the ground was not that uneven and they were able to pick up speed quite fast. With loud whoops and cheers, the cavalry forces split off to the sides, eager to complete the pincer movement. Last of all came thousands of peltasts and thureophoroi, their weapons raised in readiness. Theirs would be the final job, to charge into the Roman lines after the chariots and horsemen had smashed them apart, and prevent any attempt to regroup.

The fear among the legionaries grew palpable, and again the Twenty-Eighth began to waver, despite the officers' muttered reassurances and threats. More centurions moved to stand in the front rank, and the standard-bearers lifted their wooden poles for everyone to see. The tactic helped somewhat. No one ran – yet. Men looked nervously to their comrades, muttered anxious prayers and eyed the heavens. They were all about to die: chopped apart by the chariots or cut down where they stood by the horsemen. Where in the name of Hades was Caesar?