‘I’ve made it this far,’ he pointed out. ‘No sense in turning back now.’

When he made it to the top of the stairs he saw that they were standing in her bedroom. The bed was neatly made, Eustace curled firmly nose to tail on the coverlet. What drew his attention, though, was the ceiling. It had obviously once been papered, but most of this had been torn away to reveal a small attic entrance hatch.

‘Oh,’ Toby said. ‘I never realized you could get into the roof space.’

‘No one did,’ Rachel said quietly. ‘Or at least, no one but Cullen.’

‘What do you mean?’

‘The whole thing was hidden until recently,’ she told him. ‘I think Cullen boarded it up before I moved in five years ago, but actually, I think it had been a secret ever since this place was built. He told me once that the lighthouse had been sealed up for more than a century when he decided he wanted to live in it. This hill, the gatehouse, the cottages and the lighthouse were the only parts of the original estate that the MacDonald family still owned by then, and the lighthouse was never used. He and his mother lived in the gatehouse when he was growing up and then, when he was old enough, Cullen decided he would live in the lighthouse. I think he found what’s up there when he started working on it back in the sixties and he kept it completely secret… until he tried to tell me about it just before he died.’

‘Wow. What is up there?’

In answer, Rachel went to a stepladder that had been folded against one wall and positioned it beneath the hatch. Climbing it, she pushed back the trapdoor into darkness and reached up into the void to pull down a wooden loft ladder, which descended smoothly as Rachel removed the stepladder. She looked at him quizzically.

‘Think you can make it?’

‘Just try to stop me.’

She went up first, and as Toby began his own ascent, a cold white light swelled into the darkness above. The rungs were hard on him – there was no choice but to put all his weight on his bad leg in order to haul himself up, but he persevered.

Rachel was waiting for him at the top and crouched to give Toby a hand up. Once he’d made it, he stood still for a moment, looking around.

‘I don’t understand,’ he said, his voice echoing a little. ‘What—’

‘It’s a camera obscura,’ Rachel said. She pointed up to the apex of the dome, where a small hatch connected to a rod and a handle that ran down to their level. ‘There’s a mirror and a lens up there. When it’s open, it projects an image onto the viewing surface.’ She indicated the table.

Toby tried to take this in as Rachel went on.

‘It’s the same age as the tower. It must be. It was built as part of it, between 1812 and 1815.’