In the square, Hurricane was forming up a company of shield lines; five lines deep, ten men in each, standing shoulder to shoulder. They wore thick gauntlets, and padding on their left arms.

Along their lines the heavy rectangular shields reached down to the ground with little space under them; their ground spikes had been unscrewed. Each had one flat edge and the other edge curved into a hook along its length, so they clipped together loosely into a flexible continuous wall without gaps or overlaps that an Insect claw can pin together.

At the far side of the square, under the direction of the Macer, squads of infantry were dispatching dying Insects with heavy lead mallets, their handles one and a half metres long. They looked as if they were breaking rocks or knocking in tent pegs, but I heard the awful cracking as Insect limbs and heads gave way.

Three men with shields, one at the front and two beside him on his either side formed a triangle, running towards the gatehouse tower. A young man, sheltering between them, dragged a tiny limber cart loaded with arrow sheaves. They ran as fast as they could, reminding me of servants under umbrellas dashing across the Castle’s courtyards in heavy rain. An Insect descended towards them and the three shield men raised their shields into a roof.

The Insect landed squarely on the shields-which angled in different directions under its scrabbling feet. It slid off and the whole thing collapsed-the Insect came down in the middle, tangled in the cart and spilling arrows everywhere. Before it could right itself, the men crowded around and I saw their swords flashing as they rose and fell.

I looked down the road, seeing Tornado’s shield lines coming around the corner. They were clearing Insects before them, pushing them forwards. Insects were bracing their powerful legs on the shields’ rims, tearing at the spears, trying to crawl up the sides of the buildings, slipping over discarded wings and backing, backing, backing, as the shield wall advanced.

Tornado was walking in the gap between the first and second lines. His company was also five deep. Each line was of shield bearers and spearmen arranged alternately to thrust their spears over the tops of the shields. Those in the last line walked backwards to deal with Insects running up behind them.

Five lines isn’t many. I’ve seen this formation twenty deep when we were clearing Insects from Awian towns.

Tornado’s lines were approaching one of the radial roads. Tornado boomed, ‘Cover right junction!’

The men who heard him repeated it at a shout. It made them focus, it bound them together and those at the back heard the concerted yell. They pulled their shields in and advanced towards the street corner. Tornado called, ‘Line one, continue! Line two, stack to right!’

Behind the first line, line two began to dissolve their line across the road and instead queued up behind the right end of the first line. As they approached the junction, the men in the first line looked down the side road, saw it was crawling with Insects, and called, ‘Ten Insects, right!’

The queue of shield bearers and spearmen together dashed out from behind the first line and ran across the side road, turning as they ran to face the Insects in it. They filled the side road wall-to-wall, spacing themselves out. They slammed their shields together. ‘Ho!’

The Insects forced against the shield wall but the spearmen had them under control so quickly Tornado didn’t have to detach another line to stand behind them. He left them blocking the road and all the other lines marched across the junction.

The shield wall was left defending the junction, a vital position for the overall strategy. They shifted their weight from foot to foot, rubbed their bruised arms and hands and stared up at me. When more Insects hove into view they shouted to steady their nerves. Insects are deaf so our shouts mean nothing to them, but the men needed to reassure themselves over the unearthly buzzing.

A hiatus in the Insect storm, and I was aloft. I flew over the camp and saw the extent of the devastation. The tents outside the town wall were flattened, plastered in mud. Their drainage ditches had collapsed into brooks of sludge. Shining carapaces bobbed in the moat’s coffee-coloured water.

Around twenty soldiers were constructing a pyre outside the gate. Bodies were laid side by side next to the woodpile to be cremated. No one buries corpses in Lowespass because Insects simply unearth them.

A squad of ten women were stripping armour, belts, boots and identification tags from the bodies, leaving only the clothes on. A girl crouched, entering the details in a ledger, because armour and weapons are reissued to new fyrd and she would send any money and jewellery to the family of the deceased.

Men were looping ropes around dead Insects and dragging them out of the gate, hefting them onto a pile beyond the pyre.

A fireman was unwinding the leather pipe from his flamethrower, a cart carrying a metal cylinder of neat alcohol and rape oil. He directed the nozzle while his mate pumped the handle. They sprayed liquid flame onto the Insect carcasses. Insects are supposed to be deterred by the smell of burning chitin but I’ve never seen any evidence of it.

I hastened south to the Castle for the rest of the day and all night, rehearsing in my head what I was going to say to the Emperor. I couldn’t see the horizon, so I tried to keep the strain on both wings the same and maintained a straight line. I navigated south carefully, checking the sultry stars by my compass.

Their constellations reflected like scattered salt on my oiled wings. I have always been convinced that stars are an illusion, just like rainbows, because no matter how high I fly they never seem any nearer. The spaces between them mesmerised me and I flew on, composing my report to the Emperor in my head. I wondered what to do if Frost’s madness worsened. I couldn’t think of any way to ease the pressure on her, because she was the only one of us who really understood the dam.

I didn’t know Frost’s pre-Castle name but I have heard how she joined the Circle. She won her Challenge in 1703. She had lived all her life in Brandoch, where she founded the Riverworks Company in partnership with her husband.

Brandoch town is built on a little rise so low as to be almost indistinguishable from the rest of the drowned fenland. In Frost’s day it flourished because it overlooked the only passage through the Moren Delta deep enough for carracks. Frost and her husband laboured in the manorship’s tradition of reclaiming low-lying land from the sea which often flooded it: every one of its polder fields are man-made. They worked as a brilliant team, draining and shoring the marshy levels with dykes and long, raised roads.

Frost only sought the Castle when her husband fell ill with malaria. She realised that if she could make him immortal she had a chance of saving him. She is the most selfless of us all.

Her predecessor, Frost Pasquin, set her the Challenge of moving a fyrd division across the Oriole River using nothing but their own manpower and the materials to hand in Lowespass. Pasquin had been working at the front for too long and had lost touch with the rest of the world. He had not been aware of his Challenger’s area of expertise and he was surprised at how gladly she accepted the competition.

Pasquin took eight days to build an ingenious pontoon bridge of pine and cowhide, with a load-bearing weight enough for the five hundred men. Then it was the Challenger’s turn.

She moved the river. She surveyed it, dug a short channel and ran it into an old meander. Her husband lay on a stretcher and watched her silently, growing ever weaker while she worked day and night for five days solid. He was forbidden to help her by the Castle’s rules even if he had been well enough. The river altered its course and flowed a little south of the camp of fyrdsmen. They didn’t have to walk a step; Pasquin’s bridge was left high and dry.