Kheda glanced up at the sky. There was no hint anywhere in the heavens, no shooting stars to scar the night, no unexpected blemish disfiguring either moon.

'Father!' Sirket scrambled awkwardly up the ladder, clutching a handful of little silver cylinders. Telouet grabbed the lad's hand and hauled him bodily up on to the parapet.

Kheda snatched one of the metal tubes and began unscrewing the end caps. 'Telouet, get some light.'

'Not up here, you don't,' the slave rebuked him robustly.

Kheda stared at him for a moment before realising what he had said. 'A dark lantern then. Hurry.' He dropped to his knees, unfurling the fine roll of paper below the shelter of the battlements, squinting to make out the crabbed writing in the moonlight.

Sirket hissed in exasperation as he studied a curling slip. 'This one's from Gelim but it's in cipher.' He reached for another.

Kheda could just make out the words on the paper he held. 'Chazen boats arrive. Men, women, children. They flee unknown disaster.' He looked for the identifier at the end of the perplexing message. It had come from the central message-bird tower on Dekul.

Whatever this cataclysm may be, its ripples are lapping at the southern and westernmost of my domain's islands.

'Father.' Sirket was peering at another message in the grudging light of the dark lantern Telouet had procured from somewhere. 'Chazen Saril has seized the Hyd Rock with five triremes.'

'Why would they do that?' Kheda frowned.

'Chazen Shas made a bid for the pearl reefs east of the Andemid shoals in Daish Reik's day,' Sirket said dubiously.

'Pearl reefs have some value. The Hyd Rock is a barren lump but for a brackish pool fringed with stunted palms. That's why Daish Reik designated it a neutral anchorage for galleys travelling between our two domains,' Kheda reminded him.

'Is it an invasion?' wondered Sirket.

'Not if the whole population is fleeing.' Kheda handed his son the message he had just read. 'There was no word from Chazen while I was away, was there? No hint of a quarrel?'

'No!' Sirket insisted. 'I'd have told you.'

'And they know we could drive their warriors back into the sea without breaking a sweat,' said Telouet robustly.

'Bring that lamp closer.' The next message was unhelpfully smudged and Kheda reached out to raise the dark lantern's smoked glass slide a little.

'Chazen is invaded from the south. They bring wounded and beg sanctuary.' That was from one of the message towers on Nagel, the largest isle in the southern reach of the Daish domain. 'What's south of Chazen?' Kheda wondered aloud, slowly lowering the fragile paper weighted with such ominous words.

'South of Chazen?' Telouet looked at him with surprise.

Sirket shook his head, mystified. 'Nothing but ocean.'

Kheda handed him the message. 'Then what do you make of this?'

'Invaded?' Telouet was peering over the lad's shoulder. 'Then they'd be holding the Hyd Rock to stop whoever it is coming any further north.'

Kheda sorted through the rest of the messages. 'These all tell much the same tale: Chazen domain is beset from the south. We'd better break the coded one, even if all it says is the same. Sirket, get a bucket of embers from the kitchen cook fire. Bring it to the observatory tower.'

'Yes, Father.' Visibly confused, the youth nevertheless hurried off without question. Kheda followed him down the ladder.

'If we're going outside the gates, you wear your leggings.' Telouet thrust the heavy leather at him as soon as they set foot on the ground.

'Yes, master.' Kheda pulled the hateful things on with a grimace. The weight of the metal plates dragged at his feet as he followed Telouet towards the small postern gate on the landward side of the compound, his toes uncomfortably confined by the hard leather.

Look on the bright side. You don't have to worry about snakes if you're clumping along like some booted barbarian.

Unhampered by his own leggings, Telouet drew his swords and ran ahead to the knot of swordsmen poised by the postern.

'We're going to the augury tower,' Kheda said tersely. The chief of the guards drew the bolts on the gate and threw it open. His men rushed through, spreading out, ready to meet any threat. Telouet waited, standing between his lord and any unseen danger.

A noise behind turned Kheda's head and he saw Sirket running beside a kitchen servant who was carrying an iron bowl of live coals held tight between two lengths of firewood.

'Stay behind me.' Kheda drew his own sword.

Could this just be some ploy, to throw us all into confusion, to let some killer slip ashore unnoticed? I doubt it, but regardless, no assassin reaches Sirket while I am lord of this domain.

They ran past silent houses, shutters closed, doors swinging, a few fallen garments here and there, a scattering of broken crockery crunching under the guards' heavy-soled sandals. Relief tempered Kheda's apprehension.

Your people are safe, fled to the secret forest gullies and hidden mountain caves where everything they might need waits in sealed pots and metal chests proof against rot and insect.

The night beneath the trees was a lattice of black shadow and white moonlight. The swordsmen fanned out to either side, but met no hidden foe. Telouet scanned the path ahead, armour chinking softly as they ran. Nothing moved in the darkness beyond a few startled night birds, fluttering from swaying bushes. The solid blackness of the observatory soon loomed above them.

'Who goes there?' the tower guard challenged.

'Daish Kheda!' Several swordsmen echoed Telouet's bold declaration.

'Stand forth and be recognised.' The guard lifted a cautious half-shuttered lantern before bowing low.

Kheda sheathed his sword to unlock the door of the tower. 'Telouet, keep watch up above. Sirket, take the fire into the lower room. We are not to be disturbed.' Kheda took the guard's lantern and went in, leaving the doorway to the assembled swordsmen.

A vast circular table dominated the round room at the bottom of the tower. Sirket looked across it to his father, the glow from the embers he was holding casting mysterious shadows up on to his face. 'What do we do now?' His voice was tense, his hands steady.

Kheda smiled encouragement. 'Light the brazier.' He touched a spill to the glowing coals and went to light the lamps set in sconces around the wall. Then he took off his helm and rubbed a grateful hand through his sweaty hair.

As Sirket busied himself with a small iron fire-basket set on a slate plinth below the window, Kheda took a fine gold chain from around his neck and found the key to unlock a tall cupboard recessed into the wall. Inside, narrow boxes of iron wood with looped brass handles were packed tight. Kheda removed one and set it on the table.

Sirket looked up from tipping the live coals on to a bed of charcoal. 'Mesil was saying we should keep the keys to the ciphers inside the compound. An invader could take this tower before attacking us.'

'If invaders ever set foot on one of our residence islands, we abandon every cipher we've ever used and start with a clean sheet of paper.' Kheda unlocked the box. 'I worry about spies more than invaders in the ordinary course of things. Too many people come and go through the compound, even if they have to pass Serno and his men to do it. Up here, it's easier to see someone skulking where he's no business and only you and I hold keys to this place.'

Leafing through papers in the box, he pulled out the single sheet that hid the particular variant of the cipher agreed with the Gelim bird master woven in a cryptic riddle of its own. Kheda quickly translated the simple message. Then he did it again. He took a deep breath and studied every encoded character and its counterpart in turn. The message remained the same.