‘Pirate Captain Crow, remember.’

‘That wall is old, Conor. It could fall down. Remember the slates came off the chapel during the storm last year?’

‘What about the flag?’

‘Forget the flag and forget the goat. I’m hungry, so come down before I have you hanged.’

Conor stamped down off the wall, sulking now. He was about to challenge Isabella, say that she could go ahead and have him hanged for all he cared, and she was a rotten hostage. Whoever heard of a hostage giving the orders. She should learn to weep and wail properly instead of threatening to execute him a hundred times a day.

He was about to say all of this, when there came a dull thump from below that shook the blocks beneath their feet. A cloud of purple smoke oomphed through the doorway, as though someone had cleared a tuba.

Conor had a suspicion bordering on certainty.

‘Did you touch something?’ he asked Isabella.

Isabella was haughty even in the face of disaster. ‘I am the princess of this palace, so I am quite entitled to touch whatever I wish.’

The tower shook again. This time the smoke was green and it was accompanied by a foul smell.

‘What did you touch, Isabella?’

The princess of the palace turned as green as the smoke.

‘I may have removed the cap from the wooden box. The one with the pretty lenses.’

‘Oh,’ said Conor. ‘That could be trouble.’

King Nicholas had explained the lens box to Conor once, delighted to find that the boy’s passion for learning equalled his own.

The lenses are arranged in a very specific order, he had said, squatting low so that his own eye appeared monstrous through the first lens. So when I remove the cap, and light comes in one end, it’s concentrated by successive lenses until it can set paper alight at the other. With this little gadget it might be possible to start a fire from a distance. The ultimate safe fuse.