'Are you sure this is the place, Optio?' 'Yes, sir.'

Vespasian looked out across the river towards the far bank. Dawn had not yet broken, and the outline of trees was barely distinguishable from the night sky. The far bank was invisible, and the only sound that carried across the water was the hooting of an owl Behind the legate the trail was packed with a silent mass of legionaries, tense and alert for the first sign of danger. Night marches were the bugbear of army life: no idea of how far one had progressed, frequent halts as columns bottlenecked or simply ran into one another, and the ever-lurking fear of ambush. They were a nightmare to co-ordinate as well, which was why army commanders rarely moved troops between dusk and dawn. But the plan of attack developed by Plautius and his staff officers required that the Second Legion be across the river and in position as quickly as possible, and preferably under cover of darkness.

Vespasian had not quite believed his good fortune when news was brought to him of the discovery of a ford not two miles from the legion's marching camp. It was almost too convenient, suspiciously so, and he had questioned the optio closely. Cato, he knew from previous experience of the lad's abilities, was intelligent and cautious – two qualities the legate particularly admired – and could be relied upon to report accurately. Nevertheless, if the optio had discovered the crossing so easily then surely the Britons were aware of its existence as well. It might well be a trap. There would be little time to test this hypothesis he realised as he looked back over his shoulder to where the darkness was thinning out against the horizon. A small scouting force had to be pushed across at once. If the Britons were guarding the ford after all, the legion would be forced to march further upstream in search of another. But the more time it took to get across, the less chance the general had of co-ordinating all three attacks on the British fortifications.

'Centurion!'

'Yes, sir!' Macro snapped back from nearby.

'Take your men across the river and scout half a mile in each direction from the far edge of the ford. If you don't encounter the enemy and you're satisfied that we can cross unobserved, send a runner back to me. Best use Cato here.'

'Yes, sir.'

'If you have any doubts about the situation, fall back across the river. Understand?'

'Yes, sir.'

'And do it quickly. We haven't got much darkness left to hide beneath.' As the Sixth Century filed down the track and into the river, Vespasian passed the word down the column for the men to sit down and rest. They would need all their strength for the day ahead. Turning back to the river, he watched the straggling black mass wading across, seeming to make an inhuman din as they splashed through the gentle current. The tension only eased once the sound faded as Macro and his men reached the far side.

When the men had assembled on the river bank, Macro quietly issued their orders. He split them into sections and each one was assigned an axis of advance. Then section by section the men carefully picked their ways into the trees.

'Cato, you're with me,' Macro whispered. 'Let's go.'

With a last glance at the other river bank, silent and dark against the greying horizon, Cato turned and carefully made his way into the woods. The passage of the other sections was clearly audible at first – the cracking of twigs, the rustling of undergrowth and snagging of equipment. But the sounds gradually died away as the men grew used to the unaccustomed movement, and the sections drew away from each other. Cato did his best to keep up with his centurion without stumbling or making too much noise. He counted off each pace against the half mile Vespasian had ordered. The woods seemed to go on for ever, gently sloping upwards. Suddenly the treacherous undergrowth gave way to much more solid ground, and the trees opened out into a clearing. Macro paused and crouched down, his eyes straining to make out their surroundings.

By the faint light breaking through the tree tops Cato was able to see dim details of the ancient grove they were in. The grove was ringed by ancient gnarled oak trees, upon which had been nailed hundreds of skulls, empty eye sockets and death's-head grins surrounding him on all sides. At the centre of the clearing stood a crude altar made out of monumental slabs of stone, down the sides of which ran dark stains. A grim atmosphere wreathed the grove in its coils and both men shivered, not entirely due to the coolness of the air.

'Shit!' Macro whispered. 'What in Hell is this place?'

'I don't know… ' Cato replied quietly. The grove seemed almost supernaturally silent, even the first notes of the dawn chorus seemed muted somehow. Despite his adherence to a rational view of the world, Cato could not help being frightened by the oppressive atmosphere of the grove. He felt a compulsion to get away from this dreadful scene as soon as possible. This was no place for Romans, or any civilised man. 'Must be something to do with one of their cults. Druids or something. '

'Druids!' Macro's tone betrayed his alarm. 'We'd better get out of here, fast.'

'Yes, sir.'

Keeping to the fringes of the clearing, Macro and Cato crept past the trees with their grisly trophies, and continued through the woods. A palpable wave of relief washed over them as they left the grove behind. Ever since the Romans had first encountered the Druids, dark tales of their dread magic and bloodthirsty rituals had been handed down the generations. Both Macro and Cato felt an icy tension bristle beneath the hairs on the back of their necks as they trod softly through the shadows. For a while they progressed through the undergrowth in silence until, at last, Cato was sure that he could see lighter shades in the trees ahead.

'Sir!' he whispered.

'Yes, I've seen it. We must be close to the far tree line.'

More cautious than ever, they picked their way forward until the trees thinned out and only stunted saplings remained. They were at the top of the ridge that ran behind the river, and had a clear view down the far side and along the ridge in the direction of the British fortifications guarding the ford. Smoke from the campfires of both armies smeared the sky. To the east the sky was washed with pink and a light mist was visible down towards the river. The land to the west was still shrouded in gloomy shadows. There was no sign of any movement and Macro waved his optio back into the trees.

'Get back to the legate and tell him it's all clear, the legion can start crossing. I'll stay here a little while to make sure.'

'Yes, sir.'

'You'd better tell him what the lie of the land is like from up here. We won't be able to approach along the top of the ridge – they'd see us a mile off. We'll have to follow the river bank until we're close to the Britons and then make for the ridge. Got all that? Now go!'

Cato made his way back down the slope more quickly than they had climbed it now that the light was strengthening, revealing all the treacherous roots and brambles. Even though he kept well clear of the grove, Cato reached the river bank far more quickly than he had anticipated. For moment he panicked as he failed to see any sign of the rest of the legion on the far bank. Then a slight movement upstream caught his eye and there was the legate waving an arm from just within the trees. Moments later Cato was making his report.

'March along the river bank?' Vespasian reflected doubtfully as he surveyed the far side. 'That's going to slow us down.'

'Can't be helped, sir. The ridge is too exposed and the woods are too dense.'

'Very well. Return to the centurion, and tell him he's to scout ahead of the main force. Avoid all contact and report back on anything you see.'