The Queen’s lancers were whipping their mounts through the swarm. Each rider stopped only to pull a desperate foot soldier up behind him before turning and spurring back. They stabbed with their lances but the best weapons they had were their horses’ iron-shod hooves. The chargers’ long barding frustrated larvae trying to climb it but they could crawl under and they shot their jaws into the horses’ pasterns. Casualties were beginning to tell. Some mounts threw their riders in terror. I never saw any get up again.

I put the terrible sight behind me and turned back towards Frost. My course took me over where Lourie’s soldiers had fallen. I was shocked to find myself staring at what looked like an ancient battlefield: skulls grinned under pot helmets, bones shone through the gaps in battered armour. Every last scrap of flesh had been consumed. The bone-mounds were covered in gnarled, motionless larvae sitting with heads looking up and long tails pressed to the ground. Their thin carapaces were translucent. Some were missing legs or heads. The back of every thorax was cracked and open.

Shit. They weren’t dead larvae, they were empty shells. They were growing, shedding their carapaces within only a few hours of emerging. Five moults, the Vermiform had said. Surely all the moults couldn’t take the same length of time? At this rate, by the end of the week we would be up against a swarm of Insects greater than any we had ever seen.

I found Frost leading her group, galloping flat-out up the curving road. Gaps appeared as the stronger mounts pulled away. She lashed her horse mercilessly, obsessed with reaching the dam. The horses were dripping sweat and wide-eyed with terror at the smell of Insects.

I waved. ‘Come on! The way’s clear if you’re fast!’

They pounded on, only a couple of minutes from the dam. But as they drew closer, nearby larvae began to converge on them.

‘Look out!’

Frost ignored my warning and carried straight onto the dam’s crest, pushing her horse through the larvae as if they weren’t there. The rest followed her. I could see larvae hanging on horses’ legs by their jaws, working their way up the sides of the screaming animals. The riders flailed at them with swords, maces, gauntleted fists.

I saw the trailing horse eaten as she staggered; larvae swarmed up her legs and took her apart, stripped the skin from her side in great swathes until her guts fell out. Here and there, they laid her bare to the bone in seconds. She died squealing, trying to free her hooves from the close-packed spiny nymphs. The rider, flailing at the beasts working through his armour, threw himself over the parapet and plummeted to his death rather than being eaten alive.

With a terminal burst of speed, Frost reached her destination: the winch tower. She leapt off her horse, just short of the portcullis, shot the bolts of the iron maintenance door and disappeared inside the tower itself. The majority of her company were following. Abandoning their horses to occupy the larvae, they sprinted to safety, finally swinging the door shut behind them when it was clear there would be no more survivors. A minute later, the Riverworks Company banner jerked up the flag pole and unfurled in the breeze.

I focused on the banner, soared round and set down on the roof of the winch tower. I slipped to the parapet, between its beacon and enormous warning bell. I looked around; the timber fence along the walkway had completely disappeared. Insects had chewed the stakes down to pulpy stumps.

I looked down the faces of the winch tower. Both its portcullises were down, blocking the walkway on both sides. Both tracks were covered with larvae; they clustered around the grating trying to get through, but it was thick metal mesh and the gaps were too small. Adult Insects on the side nearest the Paperlands were chewing the bars fruitlessly.

A rhythmic clanging echoed from inside the tower. I listened carefully; it was heavy and sharp, metal against stone, and muffled as if coming from a distance, which was strange because it was directly below me.

I shimmied over the parapet and kicked a louvre through. I swung one leg inside, ducked my head under the top lintel and sat straddling the ledge, hunched half in, half out of the tower. I looked down.

The tower had no floors-it was one great, hollow dark space filled with machinery. Some candle lights flickered far below me on the ground. The scale of the crowded shafts and cogs interlaced with each other completely took my breath away.

I soon distinguished the mechanisms. On the walls below my window and opposite were fixed those needed to work the two portcullises-spindles wound with the last couple of loops of rope, and greased metal runners. The larvae’s short, whisker-like antennae poked between the bars of both gates and their jaws flashed out, tearing at the air.

The mechanism to raise the dam gate was even more impressive. Only Frost’s dedication could have designed it, and only the effort it took to have drawn it together could have made her love it as much as she did. The shafts were painted black, but their naked steel working surfaces shone with oil. The inside of the tower looked like that of a windmill in which all the wheels and beams and pegs had turned to metal.

The square floor was paved and on either side were circular, stone-rimmed holes from which thick cables ran, like ship’s ropes. They were tarred and very taut. They led up to a horizontal brass roller and were wrapped around it at both ends. It looked like the spindle used to raise water from a well, but on a massive scale. The roller was braced with girders attaching it to all four walls, secure at head height. Its vertical wheel meshed with a complicated system of gears onto a horizontal capstan carrying a chain. Lots of tackle to harness horses lay tangled, attached to the free end of the chain.

Frost was down there, dwarfed by the machinery. I could see her rounded shoulders and bandanna; she had taken her helmet off. I shouted down but she couldn’t hear me. She was bent double, peering into a square hole in the floor. Its trapdoor was open and it was big enough for one man to fit down at a time. The clanking resounded from deep underground.

Several soldiers were distributing gear piled by the walls. A big man beside Frost held a rope around his waist with both hands. The end disappeared into the shaft, and he was lowering it.

I climbed down the rough wall and ran to her. ‘Frost?’

‘Jant!’ She stood upright and stretched, her hands in the small of her back. Her face was streaked with dirt and her brigandine jacket hung open. She had stuffed her brown velvet rabbit under its fastening flap and its head nodded comically at her bosom.

‘Jant, we lost our horses. We couldn’t bring them in. We had to leave them outside and the Insect young just ripped them apart.’

‘I saw it,’ I said.

‘We needed horses to raise the gate and open the dam. Now we can’t.’ Without looking she slapped her hand onto the weighty rope. ‘See?’

‘Shit!’

‘Yes. Shit. But I think I’ve figured a solution.’

‘What are you going to do?’

She pointed down the maintenance shaft. A line of brackets bolted to the wall formed a ladder leading into the depths. A murky flicker of lamplight came and went down there.

‘Climb down and I’ll show you.’

‘I’d rather not.’

‘Come on. There are ten men crammed in the gate chamber but you can hang on the ladder.’

‘No, no. I don’t have time. I have to get back to the Emperor and tell him what you’re doing.’

The rope around the soldier’s waist tugged twice, pulling his hand, and he began hauling something up. Frost said, ‘Now we’ve made a start, I can rig a block-and-tackle to make raising the spoil easier. This is your last load doing it the hard way.’