It was four days after the hunt in the paradise that they finally set out. Despite his eagerness to get to his familia in Suania, Ballista was not unhappy at the delay. Certainly, the first day had been a godsend. The problem had been another Persian tradition. Something they had decided on drunk had to be discussed again sober to see if it still seemed a good idea – and vice versa. They had ridden back from the pool and eaten roast boar. Then, with the servants dismissed and a ring of particularly trusted clibanarii posted, they had started to drink and talked it through again. They had drunk a great deal. Pythonissa had left early – which, given nine very drunk men, had been a good thing. They had drunk through until the stars paled above the treetops. The next day, Ballista had been unable to get out of bed. He was good for nothing, except perhaps one thing. Pythonissa had visited him. While it lasted, sex gave a hungover man an unfounded sense of well being. Afterwards, of course, he felt far worse. Even on the subsequent two days, Ballista had felt washed out. He was sure he could drink less than when he was younger.

Narseh had been busy while Ballista moped about. The Sassanid prince had made great efforts to circumvent yet another Persian custom. Eastern armies – and those of the house of Sasan were no exception – liked to take their comforts with them. Huge meteor trails of wagons and carts, slaves and concubines; all manner of camp followers streamed in their wake. The length of the column was much increased, its rate of march and cohesion drastically reduced. The civilians got in the way of the warriors, and were very given to panic. To venture into the mountains thus encumbered was to invite disaster.

Issued by the authorized general and a son of the Mazda-worshipping divine King of Kings, the word of Narseh was not to be ignored. But his ukase was unpopular. Each clibanarius was to be accompanied by just one servant. Every ten light horsemen could have one servant. The hierarchical nature of Sassanid society was further reflected. Each commander of a hundred might have five servants; each commander of a thousand, ten. The prince himself – appearances had to be kept up in the sight of foreigners – would travel with one hundred. All servants were to ride. It did not have to be a horse – a donkey, mule or camel would do – but there were to be no wheeled vehicles at all. Cosis was instructed that the same regulations were to apply to his Albanians.

Ballista rode off with Maximus and Castricius to a spur of the foothills to watch the army come down into the plains. It was a warm morning; going to be a hot day. The horses stamped, swished their tails as the flies got at them. Ballista wondered whether to question Castricius about his newly claimed Macedonian ethnicity. A sophist he had once heard had claimed that we reinvent ourselves with every action, if not every thought. But publicly changing from a Gaul to a Macedonian seemed somewhat excessive.

A swarm of light horse came out from the tree line. The bowmen swooped across the grassland, wheeling this way and that out of sheer high spirits. With their bright tunics and turbans, the colourful saddlecloths of their mounts, they resembled a migration of exotic, fierce birds. Ballista estimated their number – about five hundred. It was odd watching them in amity. He remembered seeing their like on the march down to Circesium, and the fear they had induced.

Two more distinct bodies of light cavalry emerged, the numbers of each about the same as the previous division. The newcomers cantered off to right and left to flank the march. They may be deep in allied territory, but Ballista approved that Narseh was taking all precautions. He suspected the hand of the dependable Tir-mihr.

Narseh led out the main body. Above him floated a great lilac banner with an abstract design picked out in silver. The mobad Manzik carried the prince’s sacred flame, boxed for travel. Ballista was unsure about these Zoroastrian symbols. He thought each Bahram fire was lit from another; forming, as it were, an extended family.

Behind Narseh, the clibanarii rode five abreast: big men on big horses, splendid in silk and steel, bristling with lances, hung about with bow cases, maces, long swords. The column was four hundred deep – a sight both beautiful and terrible.

The baggage train was next. Ballista could see Tir-mihr and young Gondofarr spurring up and down its length, trying to chivvy it into some order. Given Narseh’s instructions, it should consist of less than three and a half thousand mounted men. It was impossible to be sure, but there seemed more. Yet many would drop out before the mountains, and at least there were no wheeled carriages.

After the camp followers came the remaining five hundred Sassanid light cavalry, with Cosis and his Albanians bringing up the rear. For the first morning of a march, it was none too bad. Ballista had seen a lot worse. He remembered old Valerian’s army straggling along by the Euphrates up towards Samosata.

‘These Zoroastrians, you have to say, have a far better afterlife than your Greeks and Romans,’ Maximus said. ‘Lots and lots of virgins.’

‘I thought that was Manichaeans,’ said Castricius.

‘Maybe them too. Either way, it is a fucking sight better than all that fluttering and squeaking in the dark like a bat. It is no wonder your Greeks will hardly fight at all.’

‘And the Romans?’ Ballista asked.

‘Nowadays, they prefer to let the likes of us do it – just proves my point,’ Maximus said.

‘I am sure it is Manichaeans,’ said Castricius.

‘But you do not know,’ said Maximus. ‘Hippothous now, he would know.’

‘I doubt it,’ said Castricius. ‘Like most Greeks, he only knows about Greek things.’

‘But he knows a fuck of a lot about physi-’

‘Physiognomy,’ said Ballista.

‘Exactly,’ said Maximus. ‘He could take one look at Castricius here, read that pointy little face and see straight into his soul – and what a horrible sight it would be.’

‘And then he could tell us why he has started pretending to be Macedonian,’ said Ballista.

‘It is a long story,’ said Castricius.

‘Are you going to tell us?’ Ballista asked.

‘Not now, no,’ Castricius said.

‘I am not sure I would want an eternity of virgins,’ said Maximus. ‘Me, I often like a woman with a bit of experience. And all the virgins are ever so willing. What about a bit of reluctance? Rip her clothes off, throw her on the bed.’

‘Stop it,’ said Ballista.

‘I am just thinking, with no concubines among the baggage, those servants are going to get very sore arses. You know what these easterners are like – obsessed with sex.’

‘Let us go down and join them.’

It was hot down on the plain, very hot and humid. It was still August, nine days before the kalends of September. They rode to the north-west, between the foothills on the right and a seemingly endless marsh on the left. They forded numerous watercourses running down from the high ground. Despite the thatched farmsteads dotted across the country, there was much unworked land. It was good cavalry country.

On the second day, they came to a place where the marsh and the hills came close together, leaving a gap of no more than four or five miles. The following day, the barriers drew back and the plain spread out in freedom. Ballista thought about Calgacus and Wulfstan and the others. If Cumania had not fallen, they had been imprisoned within its walls for thirty-one days. The fort would make a very circumscribed prison: four identical circular rooms, stacked on top of each other, each no more than fifteen paces across – dark, damp and depressing. The strain of continual vigilance, of continual fear, both robbing the goodness from the defenders’ sleep. And worse in a way for Calgacus: the views of freedom from the roof walk – the Booted Eagles and Black Vultures soaring above the crags, the Alontas river tumbling down the gorge, past the walls of the fort, and then off to the north, unrestrained by the encircling horde of barbarians through which it ran.

Maximus would smile to hear such poetic views ascribed to Calgacus. But the Hibernian might be wrong. He did not know Calgacus as Ballista did. All Ballista’s life the old Caledonian had been there – from the time childhood memories stopped being isolated incidents and pictures and became something which could, at least with creative hindsight, be ordered into a rough narrative. Beneath the wheezing, cursing and foul-mouthed muttering, Calgacus was a kind man of surprising sensitivity. Ballista was determined to get the old bastard out.

Despite the sunshine, Ballista’s thoughts took a dark turn. If he raised the siege, Calgacus would not be free, merely returned to the strange armed exile to which Gallienus had sentenced them. There was no time limit to the sentence. They had no idea where it would be served next. It was unlikely they would be allowed to return home any day soon. It was as if a capricious deity had his eye on them. Who alive was closest to a god, if not the Roman emperor? The eye of Cronus was upon them.

Yet, in a way, Ballista could not help a feeling of almost gratitude towards Gallienus. The emperor had not killed them. He had not condemned them to a small island, to pointlessly wandering the beaches of Gyaros or Pandateria. It showed practicality – the imperium was getting use out of them at the ends of the world – and a certain magnanimity of soul.

Something from the treatise On Exile by Favorinus came to mind. Something about the philosopher wandering vast swathes of territory, Greek and barbarian, seeing and hearing what happened there and, by memorizing it, making it part of an education in virtue. From what Ballista could remember, there was nothing at all in the work that even hinted at the acquisition of alien wisdom. It was all Greek.