Petronius’s words echoed back from the bare stone walls, more tunnel than passage. The air was cool deep within the fortress walls, a draught at the party’s backs making the flaming torches set in wall scones every twenty paces flutter and dance as they walked at a steady pace down into the fortress’s lower depths behind the prefect, Gurgen and Martos taking one of Osroes’ arms apiece to keep him steady on his feet. Avidus had tagged along with the party on hearing that they would be visiting the fortress’s lower depths, and his reply to the prefect’s question was wistful in tone.
‘The men that built this place certainly knew what they were doing, Prefect. Although I can’t say I’ve been surprised by anything just yet.’
Petronius laughed.
‘Don’t worry, I think you’re going to find what I have to show you entertaining. One of my brighter officers discovered it a few weeks after we arrived for our tour of guard duty. He felt a slight breeze blowing through a gap in the bricks and had the wall pulled down to reveal this rather unprepossessing passageway, running straight down to …’
He chuckled.
‘Well, you’ll see soon enough.’
After another fifty paces he stopped in front of a thick black curtain.
‘Safe to enter?’
The material was pulled back, and the prefect stepped forward into a gloomy near darkness, beckoning them forward with a ghostly pale hand.
‘Step forward five paces, then stop and allow your eyes to adapt to the light.’
A rumbling laugh from behind Marcus spoke for all of them as Lugos shook his head, invisible in the darkness.
‘What light?’
‘Ah, wait a moment and you’ll see. There are lamps in this place, just not very bright.’
Staring around himself in the gloom, Marcus realised that the prefect spoke accurately, for on either wall of whatever chamber it was that they had entered were tiny flickering sparks of light, their minuscule illumination barely enough to provide the meanest level of light to the open space, even once his eyes had become accustomed to the gloom. Julius was the first to realise what he meant.
‘The floor. It’s moving.’
Petronius laughed softly.
‘It’s moving, First Spear, but it’s not floor.’
Scaurus bent carefully, touching a hand to the glinting surface.
‘Water?’
‘Water. It’s the Mygdonius, what the locals call the Fruit River. A couple of hundred years ago some bright lad realised that the river ran so close to the city walls that they might as well do more than take water out of it. Look carefully and you’ll see how I intend to get you all out of here.’
After a moment of staring into the gloom, Martos was the first to speak.
‘Cocidius’s hairy ball sack! It’s a boat!’
‘Indeed it is, Briton. You see well in the darkness for a man with only one eye.’
The vessel was painted black, its forty-foot length filling two thirds of the chamber’s stone dock, a short mast lying flat against the planks that formed a series of rowing benches. Petronius waved a hand at it, his teeth a slash of white in the gloom as he grinned at them.
‘This is the Night Witch, gentlemen. It is an invisible boat, or at least exceptionally difficult to spot on a night like this, as I can assure from my own experience while her crew were practising with her on the river at night. I’ve stood on the riverbank and not seen her pass within twenty paces, given the right conditions.’
With a flash of insight, Marcus understood the reason why they had been delayed in leaving the fortress for three days.
‘The cloud …’
‘Exactly. There is no moon, nor any starlight. On the river you will be a black hole, visible only to the keenest of eyes set to look for such a thing. And trust me, I doubt that there’s going to be a single man looking at the river when you pass the enemy defences, given what I have planned. And now you all need to keep very quiet, we’re about to open the river gate.’
He called out a soft command, and with a slow, low-pitched rumble, a section of wall began to slide across the chamber’s face to reveal a gradually expanding rectangle of blackness. Avidus whistled softly, the professional envy evident in his voice.
‘Building this must have been some undertaking. That piece of stone has to weigh tons …’
‘It’s a deception, Centurion. The door is no thicker than the deck of this boat, but it has been coated with thin stone tiles carefully crafted to resemble the walls to either side. When you consider that it can only be seen through the branches of the thorn bushes that surround the fortress, and that it is less than ten feet high, you’ll understand why it’s almost invisible from the river’s other bank, and utterly undetectable from the distance at which our bolt throwers have kept the enemy lines.’
A dozen men in black tunics filed into the chamber through a low arched doorway and climbed carefully aboard the boat. At a signal from the prefect, a soldier handed each of the party a dark leather hide.
‘As I said, you will be a dark hole in the river, but only if you take the right precautions. Once you are out of the fortress you must keep low in the boat, and keep those hides over you. One flash of pale skin will betray you to the watchers.’
Martos leaned forward, his disfigured face barely less than terrifying in the half-darkness.
‘Watchers?’
Petronius shrugged.
‘Of course. No besieging force is going to ignore the risk that the defenders might attempt to send a messenger out by means of the river, especially as this is the one time of the year that it’s sufficiently full to be navigable. There will be men on either bank of the Mygdonius, set to watch for such an attempt, I’m sure of that. And if they spot you then your mission will be doomed, because even if you get past them, you will be hunted down by the enemy cavalry once the sun rises. The river takes many turns on its way south to the Euphrates and you will never outrun a swift horse even with the flow at your back.’
The Briton frowned.
‘If I were set to watch a river in the darkness, my first thought would be to light a fire and illuminate the river. How can we pass unnoticed if the water is lit from either side?’
Petronius grinned back at him, quite unperturbed by the prince’s scars.
‘Ordinarily it would be impossible. But I think that they’ll have more important matters on their minds than looking for boats when you pass.’
He nodded to the boat’s master, a villainous-looking soldier with a face that rivalled Martos’s for scars.
‘On your way, Thracius, and remember to wait until the entertainment starts before attempting to pass the siege line.’
The party stepped down into the boat, the dozen-strong crew muttering curses when Lugos boarded, his every movement causing the boat to rock until he was seated, with the express order from the boat’s commander not to move until they touched shore again. With their passengers aboard, the crew eased their vessel away from the stone quay, pushing gently with their oars to launch the boat slowly out into the short channel that connected the hidden chamber with the river.
‘Lie down. And remain silent!’
Marcus obeyed the master’s hissed command, flattening himself against the wooden planks as they slid into the shelter of the massed thorn bushes that covered the hidden waterway. Jerking as the first thorn stabbed at the skin of his leg, his muffled grunt of pain drew a glare and a fierce whisper from the closest of the crew, already sheltering from the bushes’ fierce assault under his own hide.
‘Use your leather!’
Diving under the heavy sheet of cow skin he felt the myriad tugs at the thick hide’s surface as the boat eased through the heart of the thorny camouflage, then there was a pirouette by the boat’s bow as it emerged into the river’s swift-flowing stream. Lifting the leather to peek out from beneath it, he found himself staring out across the plain to the east of the fortress, on the river’s far side, at the distant light of picket fires that marked the Parthian line stretching around the fortress city.