As their ice raft got closer and closer, the fortress grew in stature and menace, making Bianca feel very small. The building’s foundations appeared to spring from the rocks and the ice. It didn’t look like a man-made structure.

Casper pointed to a natural jetty, a rocky spur far below the fortress. ‘That’s where we’ll land.’

Monodon was heading straight towards it. When they were close, the narwhal kicked his tail fin and the front of the ice raft lifted, beaching on the rock.

Bianca craned her neck, studying the route up to the castle. They’d have to pass along an icy ledge to a narrow path through the rocks that led to the snowy hill on which Snow Haven sat. ‘I can do this,’ she whispered to herself, and jumped off the raft, scrambling over the rocks to the ledge. Pordis followed her.

‘Casper, you don’t have to come,’ Bianca said as he leaped off the ice raft too.

‘I know,’ Casper replied, glancing up at the looming fortress, ‘but I can’t watch you go in there alone.’

Bianca smiled at him gratefully.

‘We’ll be back as soon as we can,’ Casper said to Monodon, and the narwhal blew a jet of water into the air through his blowhole. ‘Don’t worry. We’ll be careful.’

Climbing up the snowy ledge towards the base of the castle was hard going. They progressed in silence until they reached a gap in the cliff where a narrow path stretched beneath an overhanging crust of snow. Icicles, like translucent tigers’ teeth, hung precariously above their heads. Bianca and Casper crept along holding their breath, lest their movement loosen the icy darts.

In Winterton, there had been music and laughter, but out here all Bianca could hear was the whistling wind and crashing waves.

When they finally reached the base of the snowy slope up to the fortress, Bianca got down on all fours to climb it. A strong wind pushed against her, so that she kept sliding back down. Casper tried, and met the same fate. If she hadn’t known better, Bianca would have thought the wind was working against them, pushing them down on purpose.

‘Get behind me and hold on,’ Pordis said.

The reindeer was surer footed in the snow and less affected by the wind. Bianca and Casper each had a hand on the reindeer’s back as they struggled up the snowy slope.

As they climbed higher, Bianca felt the wind getting stronger, but she refused to let it beat her. She leaned closer to the ground, digging her fingers into the snow, sheltering behind Pordis’s flank.

Despite the wind’s best efforts, they reached the top, and to Bianca’s surprise, once they were away from the edge, it died down completely.

‘That was hard work,’ Casper said, pausing to catch his breath.

‘We made it, though,’ Bianca said triumphantly, turning towards Snow Haven. The fortress had no gates. It didn’t need them. Beyond a plain of snow, the entrance, an immense archway, was waiting to swallow them.

‘Something’s coming,’ Pordis warned, nervously taking a step backwards.

A terrifying thunderous noise shook the earth beneath them. Bianca’s knees buckled, and she fell to the ground beside Casper as two great pillars of snow rose up before them. At first, they had no recognizable form, but the shoulders grew broad and a round, helmeted head appeared on each pillar.

‘Snow golems!’ Pordis exclaimed.

‘What are snow golems?’ Bianca’s voice came out in a frightened squeak as spears of ice grew in the creatures’ hands.

‘Snow golems?’ Casper looked alarmed.

‘Guardians,’ Pordis said. ‘Soldiers.’

Bianca and Casper held on to one another as they got to their feet.

An eerie blue light sparked in the eyeholes of each snow golem. They levelled their weapons at Bianca’s chest. ‘State your business,’ came their ghoulish whisper.

Bianca swallowed and, trying to sound cheerful and light-hearted, asked, ‘Are people allowed into Snow Haven to visit guests?’

The snow golems didn’t answer. Nor did they stand aside or lower their weapons.

‘I guess that means no,’ Bianca muttered, glancing at Casper.

‘We heard the palace is getting dirty,’ Casper tried. ‘We’re the cleaners. We’re here to give it a bit of a polish.’

There was no response from the snow golems.

Bianca suddenly darted forward, but they blocked her path with their spears and growled until she stepped back.

‘Aaaarrrgghhhh!’ Casper suddenly yelled as he ran at one of the golems.

The golem disintegrated and Casper fell face first into the snow. The second golem pointed his ice spear at Casper and growled at him until he’d returned to Bianca’s side.

The disintegrated golem re-formed in front of them.

‘That was brave,’ Bianca whispered.

‘Thanks,’ Casper replied, shaking his head and spitting out snow. ‘How are we going to get past them?’

‘Any ideas, Pordis?’

‘I have not.’

Every one of Pordis’s muscles was tensed for flight, and Bianca realized that the reindeer was scared.

‘Hey,’ Bianca called to the golems. ‘Do you know who I am? I’m Bianca Albedo.’

‘What are you doing?’ Casper hissed.

Both golems’ heads turned, and they fixed their glowing blue eyes on her.

‘Tell Jack I’m here. Quilo, Pitter and Patter are all looking for me. They want me thrown in the Snow Haven dungeons. Casper –’ she pointed at him – ‘brought me here. I will end up in that castle whether you let me walk in myself, or we wait until Jack orders you to take me in.’

The golems looked at each other, and the blue light in their eyes faded for a moment, as if they were sending a message. Then it flared again.

Bianca stood before them with her chin raised and her hands on her hips. ‘So, are you going to let us in or what?’

The golems bowed their heads, stepping aside. Then they disintegrated, caving in on themselves, once again becoming heaps of snow.

‘That was weird,’ Casper said, staring at them. ‘Now I’m really nervous.’

‘Me too. We no longer have the advantage of surprise,’ Bianca said, clambering over the heaps of snow. ‘Come on. The sooner we find Finn, the sooner we can get out of here.’

Dusting off her hands, she stared up at the icicle fortress. The building was a Gothic basilica of ice, with countless lofty towers that tapered into sharp spikes. The mottled exteriors were dimpled like white honeycomb. The walls made her think of wax dripping from melting candles.

‘It’s not too late to turn back,’ Casper said nervously.

‘It is for me,’ Bianca said. ‘Finn is in there. He has to be.’ She wondered if she should tell Casper about his freezing heart, but knew that she couldn’t. She wished she didn’t know about her own.

‘In we go, then,’ Casper said, without moving.

‘Something inside is in pain,’ Pordis said, trotting forward to lead the way.

The entrance was a yawning archway. On either side were columns as large in height and girth as the trees of Firfrost Forest. Pordis entered it first, the sound of her hooves echoing in the grand porch. There came a horrible grinding noise as four barred walls rose up around the reindeer and a roof dropped down, capturing her in a cage.

‘Bianca!’

‘Pordis!’ Bianca cried, running forward. ‘NO!’

There was a clanking sound as the cage lifted high into the air, suspended from the roof of the porch by a chain of ice.

‘Pordis!’ Bianca cried again, jumping up, but she could not reach the cage. She and Casper urgently searched about for a lever or switch that might release it, but found none.

‘It was a trap!’ Bianca gasped, sinking to the floor. She was trembling and on the brink of tears, but refused to cry. She needed to be brave.

‘My Bianca, I am not hurt,’ Pordis reassured her.

‘Pordis, I don’t know what to do!’

‘Yes, you do. You must go on without me,’ Pordis said. ‘Find Finn.’

‘But you are my herd,’ Bianca said with her mind’s voice.

‘And so is Finn,’ came the reindeer’s reply.

‘I’m sorry.’ Bianca’s eyes filled with tears.

‘Don’t be sorry. Be brave, my Bianca.’

Taking a deep breath, she took Casper’s hand and got to her feet.

‘Pordis, once I’ve found Finn, I’ll come and get you.’

‘I know you will, my brave Bianca.’

‘Are you all right?’ Casper asked, looking at her with concern.

Bianca pressed her lips together and nodded.

‘We’ve got to be careful.’ He looked around. ‘Who knows what traps are waiting for us in this place.’

‘Which way should we go?’ Bianca wiped her eyes.

‘Let’s try this way,’ Casper said, walking through a smaller archway into a grand lobby. The ice walls and floor inside the palace glowed with light, and the whiteness was blinding and disorientating. Smaller arches led to a labyrinth of passageways on all sides of the room.

‘Which one do we take?’ Bianca asked, looking around.

‘Well, I guess we just pick one. Are you feeling lucky?’

Bianca shook her head.

‘OK, I’ll choose.’ Casper peered down each of the passages in turn, finally pointing to one on his left. ‘This way.’ He smiled at her, then marched confidently through the archway.

Bianca moved to follow him, but Casper suddenly cried out as the ground beneath him crumbled, and he fell.