A handful of men heeded the order and raised their shields round Castor and the cohort's standard. More men joined them, roughly jostled into position by Septimus as he made for his commander. Once there were perhaps fifty men formed into a tight circle, with shields raised, Castor shouted the order to retreat along the track towards the cliff. They fell back slowly into the dusk, leaving their wounded comrades who pleaded desperately not to be abandoned to the Parthians. Castor steeled his heart.There was nothing he could do for the injured. The only shelter left to the survivors of the cohort was the partially built fort on the cliff. If they could reach that then there was a better chance of making a final stand. The cohort was doomed, but they would take as many of the Parthians with them as possible.

The small band of auxiliaries reached the foot of the cliff before the enemy realised their intention and came after them in earnest. Horsemen rode out of the darkness to loose their shafts and then reined in and steadily notched and aimed more arrows once they realised there was no further need for hit and run tactics. As the cohort edged up the track they presented a narrow target to the enemy, and a solid wall of shields protected the rear of the small band of survivors as they climbed back up to the construction site. The Parthians followed them, as closely as they dared, shooting arrows the moment a gap opened in the shields. As they realised the futility of trying to shoot through the shields they switched their aim to the unprotected legs of their quarry, forcing them to crouch low and slowing them down as they toiled up the track. Even so, five more men were injured before the track evened out and the small column of auxiliaries reached the perimeter of the site. Up on the cliff the wind was still keen, but they were at least free of the clouds of dust and could see clearly over the billowing sand that blotted out the surrounding landscape.

Leaving Septimus to command the rearguard, Castor led the rest in through the foundations of the main gate. The walls were too low to keep the Parthians out of the fort, and the only place the auxiliaries could make a stand was at the nearly completed watchtower in the far corner of the fort, on the very edge of the cliff.

'This way!' Castor bellowed. 'Follow me!'

They hurried across the maze of straight lines of rocks that marked the locations for the buildings and thoroughfares planned for the fort. Up ahead the bulk of the watchtower loomed against the star-scattered night sky. As soon as they reached the timber-framed structure Castor stood by the entrance and waved his men inside. There were barely more than twenty with him and he knew that they would be lucky if they survived to see the next dawn. Ducking inside, Castor gave orders for the men to man the platform above the tower and the window slots on the floor above the entrance. He kept four soldiers with him to defend the entrance as they waited for Septimus and the rearguard to catch up with them. There was only a brief delay before several dim figures burst through the uncompleted gatehouse and raced towards the watchtower. Moments later a wave of enemy warriors appeared and chased after them with cries of triumph.

Castor cupped a hand to his mouth and shouted. 'They're right on you! Run!'

The men of the rearguard were weighed down by their armour and already exhausted from the day's labour, and they stumbled across the site. One tripped on a loose rock and tumbled to the ground with a shrill cry, but not one of his comrades even paused to look back, and moments later he was engulfed by the wave of Parthians surging towards the watchtower. They swarmed over the fallen auxiliary for a moment, hacking and slashing at him with their curved blades. His death brought his comrades just enough time to reach the watchtower and they piled inside, lowering their shields as they gasped for breath. Septimus licked his lips as he forced himself to straighten up and report, chest heaving.

'Lost two men, sir… One back on the track, and the other just then.'

'I saw.' Castor nodded.

'What now?'

'We hold them off for as long as we can.'

'And then?'

Castor laughed. 'Then we die. But not before we send at least forty of them ahead of us to line our path to Hades.'

Septimus forced himself to grin, for the sake of the men watching the exchange. Then he glanced over Castor's shoulders and his expression hardened.'Here they come, sir.'

Castor turned round and raised his shield. 'We have to hold them here! Form up!'

Septimus stood at his side and the four men raised their spears ready to thrust over the heads of the two officers. Beyond the entrance the dark mass of the Parthians charged across the rubble-strewn ground and hurled themselves at the shields blocking the door. Castor braced himself an instant before the inside of his shield lurched towards him under the impact. Then he dug his iron-shod boots in and thrust back, punching his weight behind the shield boss.There was an explosive gasp as the blow struck home. Over his shoulder the sharp point and shaft of one of the auxiliaries stabbed out and there was a cry of agony from outside the watchtower. As the spear was drawn back a flicker of warm droplets spattered across Castor's eyes. He blinked them away as a sword blow hacked against the outside of his shield. Beside him, Centurion Septimus pressed his shield forward into the mass of the enemy crowding the entrance and thrust his sword at any exposed flesh he could see between the rim of his shield and the door frame.

As long as the two officers stood their ground and were supported by the men behind, ready to stab out with their spears, the enemy could not get in through the entrance. For a moment Castor felt his spirits rise as the fight began to go their way for the first time.

Too late he sensed the flicker of movement low to the ground just outside the entrance as one of the Parthians crouched and swept his blade beneath the rim of Castor's shield. The edge of the blade cut deep into his ankle, severing leather, flesh and muscle before it fetched up against bone. The pain was instantaneous, like a red-hot bar thrust into the joint. Castor staggered backwards with an explosive cry of pain and rage.

Septimus glanced back quickly, seeing his commander slump to one side of the entrance. 'Next man! Into line!'

The nearest auxiliary, crouching low to protect his legs, pressed himself forward, alongside Septimus, as his comrades thrust their spear tips at the enemy in a flurry of attacks to drive them back from the entrance. Then all at once there was a shout of alarm from the darkness and the crash of heavy masonry outside the watchtower. As Castor leaned round the frame to look he saw a piece of dressed stone smash down on to the Parthians, crushing a man's head as it drove his body to the ground. More rocks and stones fell on the attackers, killing and maiming several before they could scramble back across the site to a safe distance.

'Bloody marvellous,' Septimus growled with pleasure at the sight. 'See how they like being hit without a chance to fight back. Bastards.'

As the enemy moved out of range the barrage of stones tailed off and the sounds of combat gave way to the jeers and whistles of the auxiliaries in the watchtower, and the moans and cries of the injured men in front of the entrance. Septimus took a last glance outside before he motioned one of the men to take his place. Leaning his shield against the wall he knelt down to examine Castor's wound, straining his eyes to make it out by the wan glow from the starry heavens shining through the entrance. His hands gently probed the injury and felt the shards of bone amid the mangled flesh. Castor sucked in a deep breath and clenched his teeth as he fought back the impulse to cry out in agony.