Conor remembered thinking at the time that you could leave the box by the window and have it light the fire for you each morning, a chore that he was none too fond of.

And now Isabella had removed the cap.

‘Did you move the box?’

‘Mind your tone, commoner!’

Commoner? Isabella must really be terrified.

‘Isabella?’

‘I possibly placed it on the table, by the window to see the colours passing through.’

Obviously the device had caught the afternoon light, releasing the power of the lenses into the king’s laboratory, filled with the fertilizer, jugs of fuel and various explosive materials. The concentrated light had obviously landed on something combustible.

‘We have to go,’ said Conor, all thoughts of Captain Crow forgotten. He was no stranger to the power of explosives. His father was in charge of the Wall defence and had brought Conor along on a trip to collapse a smugglers’ cave. It was a birthday treat, but also a lesson to stay away from anything that went boom. The cave wall had collapsed like toy bricks swatted by a toddler.

The tower shook again, several floor blocks rattled in their housings, then dropped into the apartment below. Orange and blue flames surged through the holes, and the snap and grind of breaking glass and twisting metal frightened the two children.

‘Up on the wall,’ said Conor urgently. ‘The floor is falling.’

For once, Isabella did not argue. She accepted Conor’s hand and followed him to the lip of the parapet.

‘The floor is a foot thick,’ he explained, shouting over the roar of the flames. ‘The parapet is four feet thick. It won’t break.’

The explosions went off below like cannon fire, each one issuing different odours, different colour smoke. The fumes were noxious, and Conor presumed his own face was as green as Isabella’s.

It doesn’t matter if the parapet holds, he realized. The flames will get us long before then.