Billy edged his way along the base of the Keep until he reached the west tower. A strong wind was blowing, and a heavy blanket of cloud made the night seem darker than ever. Now he was up close to the Magician’s quarters he wasn’t sure it was such a good idea, but he was determined to go through with his plan. It might be his only chance of ever getting home.

‘How do we g-get in?’ Tom asked in a nervous whisper.

Billy shrugged. ‘No idea,’ he said.

He carried on around the tower, almost missing a low metal door in the darkness. It felt rusty and warped out of shape. He gave it a shove, and it opened with a squeak.

‘I don’t believe it,’ he whispered, pushing the door some more. ‘It’s not even locked!’

He stepped inside and there was a loud scrabbling as a swarm of rats ran for cover. A burning torch cast a trembling light across a litter-strewn floor and revealed a spiral staircase curving up into the gloom. Billy took the torch from its bracket, and with a nervous grimace towards his friend, he began to climb. Tom followed close on his heels.

The higher they climbed, the nearer they got to the Magician’s private rooms, and Billy felt the air crackle with a terrible power. It made his skin prickle, as if a thousand ants were crawling over him, but he swallowed his fear with a gulp, took out Fleetfoot’s sketch and studied it in the light of the torch.

‘Only one more flight and we should find the door through the wall,’ he whispered. He was finding it harder to breathe now, as if some great weight was pressing down on his chest. When they reached the next landing he scanned the stonework, looking for a hatchway, but couldn’t see any sign of one.

‘It’s got to be here somewhere,’ he said, starting to worry. Maybe Fleetfoot had been crazy, after all.

‘I c-c-can’t see anything,’ said Tom, peering over his shoulder.

Billy circled the inside of the tower, brushing aside cobwebs and moss. Just as he was about to give up, he uncovered a narrow slot in the mortar, like a letter box. He put his hand inside and felt a metal chain.

‘This must be it!’ he exclaimed. He grabbed the chain and tugged hard.

There was a dull clunk, and with a grinding noise a stone door slowly opened in the wall.

‘Yes!’ exclaimed Billy, his eyes shining with nervous excitement. He leaned out – it was a sheer and dizzying drop to the cobbled lanes below, but to his left was a narrow ledge running around the Keep, just as Fleetfoot had described. He propped the torch against the wall.

‘OK?’ he asked Tom. ‘You’ve still got time to pull out.’

‘N-n-n-no, I’m with you,’ said Tom, though he looked as nervous as Billy felt.

Billy climbed up into the opening and stepped out onto the ledge. It was windy and a heavy downpour had made the stone slippery. Not daring to look down, he edged slowly along until he came to a large slab standing slightly proud of the Keep’s wall.

‘This must be the secret entrance,’ he shouted above the wind.

Tom nodded, too scared to speak.

Billy gave the slab a push and it sprang forward on stone runners. He climbed inside and helped his friend in after him.

‘Thanks,’ the boy gasped, with a sigh of relief. Then the slab closed silently behind them, and everything went black.

‘Oh, brilliant,’ whispered Billy sarcastically. He couldn’t even see his hand in front of his face. Then a faint yellow glow appeared in the darkness.

‘W-w-what’s that?’

The light started to grow and spread. After so long in the dark it was painful to look at, and Billy had to shield his eyes. But, gradually, he became used to the glare and with a gasp of astonishment saw a wide landscape of rolling fields spread out before him. It seemed to go on for miles, all the way to a distant horizon and a clear, golden sky. A small river sparkled as it wound its way before a dense wood, and the air was full of birdsong.

‘Oh, w-w-wow! It’s amazing,’ cried Tom.

‘Incredible,’ said Billy, basking in the warmth of the sun. It felt so good after the gloom and damp of the fortress he really wanted to lie down and soak up the golden rays for ever. But the air tingled with magic, like the air in the Bright Room, and he knew it must be an illusion and that they were still inside the Keep.

‘Come on, we have to move on,’ he said, realizing they would have to fight against the Magician’s magic all the way.

‘But w-w-where do we go?’ asked Tom.

‘Good question,’ said Billy, but as he spoke he heard a faint rumble in the distance and a thin, jagged mountain began to rise between two conical hills on the horizon. It was shaped like a crooked finger. ‘That’s got to be the way,’ he said excitedly. ‘Look! It’s beckoning us on!’ Perhaps it was a trick of the light, but the finger of rock seemed to be moving.

‘I d-don’t like it,’ said Tom, his large eyes growing even wider. ‘It might be a t-t-trap.’

‘Maybe, but I don’t know where else to go,’ said Billy, and they set off down the grassy knoll towards the mountain. The sun was shining, the air was fresh and clear and everything seemed perfectly normal. And then, in an instant, it all changed.

The ground beneath his feet turned soft and spongy, like marshmallow.

‘What’s going on?’ Billy shouted. He was sinking into the ground, and the more he struggled the deeper he sank. Soon he was up to his knees in gluey, clinging mud. Tom was sinking too.

‘It was a trap!’ Tom cried. ‘Help!’

‘Over here!’ yelled Billy. There was a wide slab of rock protruding from the ground, and he managed to pull himself onto it. Tom struggled towards him, but he was sinking ever deeper. Then the air began to congeal. It became as thick as syrup and they could only move in slow motion. It was like a bad dream.

‘Heeelp, B-B-Billeee!’ cried Tom, his voice drawling like a slowed-down recording as he sank up to his waist.

‘Don’t paaanic!’ Billy hollered, but the ground bubbled and popped and Tom sank up to his chest.

Without thought for his own safety, Billy slid back into the treacherous mire. He immediately started to sink, but grabbed hold of the edge of the slab to steady himself. Then he reached out with his other hand for Tom and started to pull with all his might. He pulled so hard he thought he might break in two, but inch by inch he managed to drag Tom towards him. Eventually, his friend was close enough to grab hold of the rock.

Billy had sunk up to his waist, but with a monumental effort he turned round in the viscous mud and dragged himself back onto the slab. He staggered to his feet, took Tom’s hands and pulled his friend up beside him. As he did so, the air thinned and they could move normally again.

‘Thanks, Billy,’ said Tom in an exhausted whisper.

Billy nodded and collapsed back on the rock, taking in great gulps of air. As his rasping breathing subsided, he became aware of another noise – a loud rushing roar.

‘What’s that?’ he asked, and sitting up gave a cry of surprise. The soft, sticky ground had turned to liquid and now a vast stormy ocean surrounded them. ‘Watch out!’ he shouted, as a strong swell washed across the rocks, swept him into the torrent and carried him out to sea.

Billy struggled to stay afloat as the swell grew into a wave, and grew and grew again, lifting him up onto its foaming crest. His stomach flipped over and over, as if he were riding a giant roller coaster.

‘Tom!’ he cried, desperately looking for his friend as the towering wave carried him forward like a powerful juggernaut, high in the air. Then it pounded him down, and he sank deep below the waves. All he could see was green foaming water and his chest began to burn from lack of oxygen. Summoning all his strength he swam upwards, against the swirling current that was trying to force him down. With a rasping gasp, he broke the choppy surface and looked for his friend.

Tom had already worked a hard shift in the Pits before being rescued, and was floundering helplessly nearby. Billy grabbed him under the arms and headed for the distant shore. He could feel his own strength gradually ebbing away, but he wasn’t going to abandon his best friend, and Billy finally pulled him onto a golden sandy beach.

‘You saved me again,’ spluttered Tom.

‘No problem,’ panted Billy, watching as the vast sea sank back into the ground, as if it was a huge sponge. Somehow Billy’s clothes became instantly dry, and he knew for certain he was inside an illusion. But each challenge was making him more exhausted than the last.

‘Look where we’ve landed!’ he said, with relief. About a mile away, the great black mountain rose like a giant shard of jet amongst a line of gentle, sugarloaf hills.