Dak followed her father to the far end of the room. Once there, he shone his hand-light on the wall. The long room was empty except for four tall urns, each one painted with a bright pattern, their long necks heavily stoppered and bound with waxed twine. Its walls were of precisely cut stones like the rest of the cellars, but the wall that her father illuminated with the liquid light was made of welded metal plate.

"What is that?" asked Dak.

The wall's single central plate, three metres high and two wide, was filled by a circular design of concentric rings, with a six pointed symbol at its centre.

"It is a moon lock," explained her father, and Dak was glad to see the old look of pleasure back on his face.

There had once been a time when the joy of revelation had constantly suffused her father's features. He would often take her on tours of the fortress to point out things of interest, be it an area of well-crafted architecture, or a particularly clever piece of engineering design. He had once delighted in showing her its countless wonders, and they would spend many hours in the day following its passageways and climbing its towers, but those excursions had ended when her mother had died.

After that day, her father had fallen into a low brooding state, seeming to lose interest in everything except for sitting in his chair and staring out across the rooftops of the workshops. It had only been Sir Galder's commission to make him new armour that had broken him from his stupor. With the work done, Dak had been concerned that her father was returning to his brooding, but it seemed he was not. Dak was delighted that their tours of the fortress had resumed, though she suspected their true purpose was to keep her away from Tahlia.

On the afternoon that the rains had come, she had turned up at the workshop door, dripping wet and imploring her father to leave the dry warmth of his bed to go out into the wet and rescue her friend from the river. He had done so grudgingly, but only after she had told him that Maddock shared her friend's predicament, and when he had returned he had been stoked with anger.

"You will not be seeing that girl again!" he had raged in a most uncharacteristic way. "If you are keeping her as a friend, she will lead you to more and greater trouble. You are not to leave the Workshops without my say, and she is not to enter here. I have informed the Forge-guard Chief, and his people are now having orders to refuse her if she turns up at our gates, and if she is, by chance, finding her way to our door you are to turn her away. Are you understanding me, daughter?"

She had been unable to speak in the face of her father's fury, so had kept silent and simply nodded and then continued preparing their evening meal.

Her father's anger had slowly abated, and on the third day of the rains' battering he had taken her up to the fortress' heights, both of them draped in tragasaur tarp capes, to show her the rooftops' ornate guttering. The next day they had walked the battlements, and he had shown her the drains that channelled the water from their flat walkways. She had asked him where all the water went, so a few days later he had taken her down through the cold of the fortress' ale and wine cellars, where casks and bottles were lined in rows along long curved corridors.

"Come, touch this," her father had said, going to the convex wall and placing his hand on its metal surface. Dak had done the same.

"It is freezing!"

"Behind this wall is the reservoir core," her father had explained. "All the rains that fall on the fortress are stored in its insides. It will last the fortress through the year."

"The fortress must use a good deal of water," she had asked. "Does it not run out?"

"No. And when we get the chance, I will take you to a place and show you why."

Which was why, a few days later, they had passed down through the fortress' cavernous kitchens, which resonated with the constant rhythms of activity. Pies were being made, fruit slow stewed, bread baked, kernik seeds malted, vegetables boiled and meat roasted. The air had been spiked with the sweet smell of sugar reed being crushed, and the earthy smell of jedlea spines being ground, along with the bitterness of fire root, the smell of which had tanged fiercely at the back of Dak's throat. Huge cauldrons hung over fire pits, young boys and girls ground the herbs and spices with pestle and mortar, stirred vats of batter, and kneaded giant balls of dough.

They had passed from the kitchen's noise and heat, into the cold quiet of its cellars, to the long room with the moon lock on its end wall.

Dak looked curiously at the design.

"How does it work?" she asked.

Her father answered with a question.

"How do your lessons with Engineer Drasneval go?"

"I think they go well," Dak replied. "She seems pleased with my progress."

"And she has taught you about the cypher, yes?"

"We started studying it last year, father."

"And so do you understand its working?"

"Yes, I can perform the calculations well, but Engineer Drasneval has not yet told us about its applications."

"And I have a suspicion she will be keeping that knowledge from you for another year yet."

"She says that we have to master the cypher before she demonstrates its uses."

"And do you think you are anywhere near that level of mastery yet?"

"I think so, father," said Dak, remembering her last lesson with Engineer Drasneval.

"Confident words!" said her father. "Well, let me propose a test and I will see if you are living up to them. If I give you your numbers, and if you run them through the cypher and give me a correct answer, I will show you but one of its uses."

"That is fair," said Dak.

"Now," said her father, looking thoughtful. "We are on the fifteenth level of the fortress, so that is to be your first number."

"Fifteen," confirmed Dak.

"We are still in the confines of the keep, which is in sphere one, so that is your second number."

"One," said Dak.

"And we are in the section of the south shield bastion, which is section four, and so four is your final number."

"Fifteen, one and four," said Dak. "What gear ratio am I to use?"

"Use today's," said her father, "Which is eighteen to six."

Dak closed her eyes and calculated the cypher. Just as it had been in Engineer Drasneval's class, she found it easy enough without chamber pen and paper. Still, she again ran the cypher twice through in her head to be certain of the answer. When she opened her eyes, her father was peering at her curiously.

"The answer is forty seven," she told him.

His eyes widened.

"By Yeltov, that was quick!"

"Is it correct?" asked Dak.

"It is quite correct, daughter, and I have seldom seen it done with such speed. Does Engineer Drasneval know you have this talent?"

"Yes, father. She came to talk with you about it, but you were not well on that particular day. I think she wrote you a letter afterwards."

Her father then looked truly sorrowful.

"I am sorry, daughter; you have been neglected this past year. I must try and make it up to you by as many means as I can."

"Thank you, father."

"And as promised, I will start by showing you for what purpose the calculation was performed."

He turned to the moon lock.

"Here," he said, indicating the three outer rings. "You see that each ring is divided into fourteen sections; each section is representing a passage of the Sladin moon, being the fourteen months of the Khensis year, and each ring representing the three cycles of the Taqi moon within each month."

Dak nodded.

"And this," said her father, indicating the symbol in the centre, which was surrounded by a fourth ring, itself divided into nine sections. "Indicates one cycle of Taqi, being nine days. So, do you have an idea as to how the lock is to be opened?"

He gestured with his hand.

"Touch. See what you think."

Dak stepped closer to the wall and raised her hand to the design, running her fingers over the circles. Fourteen segments of three, each section representing one of the forty two weeks in the year. She noted how each section had a deep groove inscribed upon it, joining a point on each of the section's four sides to the centre. Sometimes three sides were joined to the centre, sometimes two or one, but seemingly never all four, and the inner most ring had no connections to the centre of the design where the six pointed symbol lay.

She placed a hand on the outer circle and gave it a tentative push, and as she had thought, the circle turned smoothly, as did the other two when she tried them so that, if aligned correctly, it would be possible to join the grooves to make a continuous design, like a circular maze.

"And your ideas, daughter?"

"What are those?" asked Dak, indicating two circular symbols, one fifteen centimetres in diameter that sat directly above the larger circle, touching its edge, and a smaller one of roughly five centimetres, which sat on the opposite edge, at the bottom of the design. The larger circle had four circular indentations around its edge, also roughly five centimetres in diameter.

"Those are the keys," said her father. "And it would also help you to know that the cypher, when given the three numbers linked to each moon lock, and the current gear ratio, will always return to you an answer between one and three hundred and seventy eight."

Dak studied the lock once more. She was used to the clever locks of the Engineers and knew how intricate they could be, like the lock on the conquest table back at home.

"I think I understand," she said after a few minutes.

"Yes?" said her father, though he looked a little doubtful.

"The three rings must be aligned, so that both keys can be moved along the track to the week in which lies the forty seventh day."

Her father smiled.

"You have a quick mind. This is why I am proud that you are my daughter."

Dak almost felt the blush that she knew must cover her face.

"So try it," said her father, gesturing towards the lock once again. "But be cautious. If your choice is incorrect we will not be passing through this door today. One chance is all you shall be having.

Dak looked at the lock.

"The forty seventh day is in the third week of the second month." She indicated the corresponding segment of the design, which was only one section from where the larger key already sat on the outer edge. It would be a simple matter to move it to the correct position, but the smaller key would have to take a longer more complex route. She did not want to rotate the rings randomly in an attempt to find the correct path, so instead she studied the design carefully, rotating the rings in her mind and picturing the route it would create. It was not easy, as moving a ring to open a path for one key would invariably close a path for the other. It took her five minutes to see the trick of it. The larger key would have to make a more circuitous route than she had first thought. She rotated the rings, until she was sure they were in the correct position, then she moved the keys along the tracks; first the larger and then the smaller, which fitted smoothly into one of the other's round indentations.

She stood back.

"That is the correct week," she said. "How do we select the correct day?"

She looked at her father,who was beaming at her, a look of wonder on his face. He pointed at the six pointed design in the centre of the lock, tapping the bottommost point. Each point had a different symbol at its tip.

"That is the fortress," he said simply. "We are in the section of the south shield bastion."

Dak understood instantly.

"Day forty seven is the second day in the third week, so..."

She gripped the symbol and turned it, so that the point representing the south shield bastion pointed to the second section on the surrounding ring.

"Are you certain?" asked her father.

Dak looked at how she had positioned the symbols, double checked her calculations, and nodded.

Her father nodded grimly in return and reached out to the hub of the central symbol, where another round design was set. He pressed it firmly, and it disappeared smoothly into the door with barely a sound.

A dull thud came from somewhere deep inside the door, followed by the whirring of gears, and four more deep metallic clacks, then the section of wall on which the design was set swung smoothly open.

Her father looked suddenly concerned.

"You must promise me something, daughter," he said.

"Yes, father," Dak replied.

"You are not to come through any door such as this on your own."

"Of course, father," she said.

"And you are not to talk to anyone about them who is not of the Guild."

"No one? Not even Maddock?"

"No one. Maddock is a good lad, but this is a thing of confidentiality."

"I promise, father."

Her father smiled, though still looked serious.

"Good," he said. "Now we can go in."

He pulled the door fully open and stepped through, turning down his hand-light until it went out. Dak was left in the darkness of the chamber. All she could see was the shape of the doorway in front of her, lit from within by a blue glow. She frowned in the darkness and followed her father.

"You see," he said.

There was a long metal passageway beyond the door, wide enough for them both to stand side by side. The wall opposite the door rose from the floor, and when it reached the height of her father's waist, curved a quarter circle away, before continuing upwards to arch over their heads.

Set up in the apex of the arc of the ceiling were three thick pipes, which followed the length of the tunnel and disappeared from sight as it curved away in each direction. High on the walls at intervals along the passageway, glass strips glowed with a blue light, and they brought the details of everything out in an odd sharp clarity.

Her father pulled the door closed behind them and it shut with barely a sound, though Dak could hear the gears spin, followed by four dull thuds as it locked.

"Surprised, daughter?" he asked.

"Yes," said Dak, looking around at the lights and the metal walls. "I was thinking it would be smellier."

"Ah, a commonly construed misconception. The sewers used to give the most effluent of stinks, but now we have their entirety sealed away behind here."

Her father tapped the curved section of wall with a knuckle. Now that Dak listened, she could hear the slosh and gurgle of water behind.

"These passages are merely being the access tunnels. The real business goes on behind there."

Her father then tapped each of the three pipes in the ceiling above in turn. Dak noticed that each was made of a different type of metal, each one being a different colour.

"Heat, clean water and light," he said. "You will learn how they all work in time, but just pretend they are not there for now. It is not what we have come to see. Now we go this way."

She followed her father along the passageway, which sloped and curved gradually downwards. As they followed it, they passed other passageways leading away from the one they were in, each either sloping up and outwards or down from the inner curving wall. All around was the distant hollow gush and drip of water.

"The water is pumped from the reservoir core, all over the fortress to be used, then it flows back through here from all the bathing and wash rooms, the kitchens and the laundry rooms, down to the waste reservoir. That is where you will find the smells. The waste reservoir is in the old sector, where the plumbing is getting a little vintage. The whole reservoir requires replacing, but we are still waiting for approval from the Higher Guild, just as soon as they have formulated how it is to be done without great disturbance to the fortress."

They entered a circular chamber where a pipe, wider than Dak's outstretched arms, entered through the ceiling and disappeared through the floor. More of the smaller coloured pipes entered the room through other doorways, to join with each other in a tangle of gauges and pressure wheels, similar in type to the ones on the barapane tank behind the forge back at home. Beside each valve, and upon each pipe, small metal plaques had been ring bolted, each one neatly stamped with an inscription.

Her lessons to date had not covered the mechanics of the fortress in detail, and her only knowledge of them consisted of the few things her mother had explained to her, along with her father's more rudimentary knowledge. She understood the workings of her father's forge, but even that was only the fundamentals of igniting and running it, which consisted mainly of regulating the pressure in the barapane integration chamber. The stamped inscriptions on the metal plaques meant very little to her. They were written in the Engineer's machine script, and though she could read the words, their meaning was a mystery.

They read 'Third Sphere Radial Loop', and 'Helix Sink One, Two and Three', and 'Outside Core Ignition Reservoir', and others just as ambiguous.

"This way, daughter," said her father, breaking her study of the pipes and valves.

He led her down a set of steps, which curved around the pipe in the room's centre, and they descended together through the floor. More blue lights lit their way down, until the pipe exited through the roof of a great chamber. The stair continued to spiral down it, now with a handrail to protect them from the drop into the darkness below. Dak could see where many more wide pipes entered the chamber from openings around the walls, and they all met with the pipe down which they were climbing, each one with a metal walkway and railings running along the top.

Her father stepped onto the giant pipe's encircling walkway. The stair continued its spiralling journey downwards into a darkness filled with the bitter smell of wet metal, and the hollow sound of distant churning water. Looking down, Dak was beset by a sudden guilty memory. She was standing again on the rain drenched riverbank, water tumbling darkly beneath her as Tahlia shouted for her help. If she ever saw Tahlia again, she hoped her friend would forgive her for her cowardice.

"This is where the old sector starts," said her father, pointing down into the darkness. "It is too dangerous for you, my daughter, so we are to take a safer route."

"Why is it dangerous?"

"The sewers are open and mostly unmarked. It is a place where we Engineers only go out of necessity, and even then only by those who understand its layout."

He led her along one of the walkways.

Suddenly, above the sound of distant water and the echoing clang of their boots, Dak heard a new noise. It was like the chittering of animals and the squeal of decayed metal being torn back and forth.

"What is that, father?" she asked.

Her father placed both hands on the rail and peered down into the darkness.

"A nest of metal crousks," he said. "It is hatching time so we would be best to leave them be."

He continued along the walkway.

"A nest of what!" Dak's voice echoed around the chamber.

Her father put his arm out to halt her progress. The metallic chittering had stopped.

"It is better to be quiet around them."

"But what is a metal crousk?" Dak whispered.

"See for yourself," answered her father. "Here comes one now. Your noise must have risen its interest."

Over the edge of the walkway ahead rose two long segmented feelers, searching the metal of the walkway by tapping and caressing its surface. Next, a dark segmented head, split in horny ridges, came into view. There were no eyes; just a dark space from where the twin antennae probed. A few spike tipped legs joined the antennae in their investigation of the walkway, and then the rest of the creature's body was pulled over it's edge. The crousk had two long rounded openings in its armoured back, from which further sets of fine feelers waved back and forth, probing the air around. Its body tapered back in segments and ended in a single black needled tail. In its entirety, it was over a metre in length and at least twenty legs bristled at the edge of its dark shell, and they each made a squealing tick as it carried itself towards them. The blue lights, high above in the chamber walls, reflected on the black surface of its back with an oily metallic sheen.

"Stay behind me," said her father. "And be quiet."

The crousk continued its inquisitive progress towards them. One of the antennae lingered on Dak's boot, tapping it a few times before the creature moved on along the walkway.

Dak let out a long breath once it had passed.

"They are no problem unless they feel threatened, and then they are very dangerous. They have teeth like a trap and can pull a man's heart out through his chest."

"Why do you let them live down here if they are so dangerous?"

"They hunt the rodents and suchlike."

Dak could see another crousk on the far wall of the chamber, crawling out of the wet shadows beneath the pipes.

"We had better leave," said her father. "They are unsettled and curious."

He led her along the rest of the walkway, into another blue lit corridor. The passageway continued upwards, but they did not follow it. Instead they took a side passage, which ended in a lift chamber. The lift itself was small and barely held the space for both of them. Her father worked the levers and they began a rattling descent downwards, seeming to fall a good distance before stopping at the lower lift chamber.

From there, her father led her through further narrow passageways and rooms with more of the complicated sets of gauges and levers, until they finally came to a wider corridor, sloping steeply downwards.

"This way, daughter."

She followed him down, and they entered a cavernous space, full of the dripping echoes of water,which set up a strange rippling harmony within the vast space. The flickering glow of blue light that had filled the rest of the tunnels was replaced by an odd green luminescence. The air was moist, and Dak felt sweat prickling her brow from the warmth that enveloped them as they entered from the cool of the tunnels. It also held a deep pungent smell that Dak did not recognise, but it seemed to fit with the close humid air.

"Come," said her father. "There is someone here I am wanting you to be meeting. Do not be afraid."