At first, the river Phasis was very broad, wonderfully calm – like smoked glass – after the Kindly Sea. On either bank, beyond a thick screen of reeds, was low, marshy primeval forest. Everything was very green, very flat. The air was humid, misty. At clearer moments, the Caucasus loomed on the left.

The river meandered in great sweeps, little archipelagos of sodden, uninhabitable islands in the bends. Nothing but the splash of the oars, the water singing down the sides of the boats, and the endless croaking of innumerable frogs. However, not all was peace. A constant impediment, if not actual danger, was the great lashed-together rafts of logs the natives floated down to sell in the city for shipbuilding. Again and again, the camarae hurriedly had to pull to the banks to avoid collisions with the unwieldy masses of timber. It was, Hippothous thought sourly, the only time their oarsmen displayed the merest hint of energy or alacrity.

By the end of the second day, another problem materialized. The silt carried down by the river created an ever-changing pattern of shallows and mud banks. A man in the prow of the lead boat probed the riverbed with a long pole. The helmsman was faced with continual choices as the channels of the river divided again and again. Not all his choices were good. Although the camarae drew little water, they ran aground with increasing frequency. Here, the double-ended form of the boats came into its own. Quite often, the rowers merely reversed their position, the helmsman scurried to the other end of the boat, and the oars pulled her off. If that did not suffice, things became considerably more fraught. The crew had to go overboard and, standing waist or even neck deep in the turbid water, manhandle the boat free. This they were most reluctant to do. Like most of the great watercourses, the Phasis bred man-murdering monsters. The travellers were told these looked like catfish but were larger, blacker and stronger; as man-eating as any in existence, as deadly as the horrors that were hauled from the Danube with teams of horses or oxen.

Each time the men came back over the side, muddy but unmolested, the Colchians would laugh, clap, break into song. One solemnly assured Hippothous that their continued good fortune was owed to all on board heeding local wisdom. Before setting out, Hippothous and the others had been enjoined to empty all their water skins and the like. To carry alien water on the Phasis was to bring the very worst luck.

Whether it should have been credited to the absence of foreign water, or to the kindly hand of a deity, none of the dark monsters made an appearance. But with the searching for a channel, the logs and the groundings, the perceived idleness of the natives, progress was very slow.

Dawn on the fourth day, and the river narrowed and the forest thinned. Signs of habitation increased: fields, orchards, isolated log huts. Small, near-naked children tended flocks. They waved as they brought their charges to water at the riverbank. To the north, the Caucasus seemed only a little closer. But to the south, the hills advanced near, rising in steep, timber-covered slopes.

The things that did not change were the dampness, the lushness and the interminable noise of the frogs: brekeke-kex, ko-ax, ko-ax. They preyed on everyone’s nerves. Brekeke-kex. None more so than Maximus – what he would not do for some fucking peace. Sat in the stern of one of the stinking camarae, Hippothous told Maximus every fable of Aesop featuring frogs that he could recall. It was like soothing a child. The barbarian enjoyed the ones where the frogs suffered and died an unpleasant death. His favourite was the one where the frogs, tired of their democratic existence, asked Zeus for a king. The god sent them a log. Unimpressed with its inactivity, they petitioned for a new monarch. So Zeus sent them a water snake, which ate them.

‘And the moral is?’ Hippothous asked.

‘Be very afraid of snakes.’

‘No.’ Sometimes Hippothous wondered if the barbarian was mocking him. ‘It is better to be ruled by an indolent emperor than an active one who is malevolent.’

‘Or,’ Calgacus suddenly spoke, ‘the obvious truth that all change is likely to make things infinitely fucking worse.’ Hippothous was not sure he had ever met anyone more gloomy than Ballista’s old Caledonian freedman.

They slept that night in a town with the happy name of Rhodopolis. There were indeed roses. The polis was set in a fertile plain. Once, maybe in the days of Hellenic freedom before Rome crossed the Adriatic and her greed impelled her to the ends of the earth, Rhodopolis had been a fine place: temples, agora, Bouleuterion; everything a Hellenic polis should have. But it was much decayed, and bore the signs of both violence and neglect. Possibly, Hippothous thought, in this case Rome was not to blame. Rhodopolis had been sited with a view only to wealth and not defence.

On the fifth day, the hills crowded close on both sides. The stream was narrow and fast, the going slow. At the time when two thirds of the day had gone, the time when exhausted labourers pray for the dark to hurry, the camarae fought their way around the last corner, and gratefully moored at Sarapanis.

Sarapanis was a neat, small village of tiled houses, a touch run down. It clustered at the foot of a steep, conical hill. At the top was the fort. Its garrison proved to consist of only sixty men. But the walls were sound and they had two pieces of artillery. The whole – fort and village – was almost entirely encircled by the confluence of the Phasis and another river. To make good the fort’s lack of a natural spring, an underground tunnel had been dug down to one of the rivers. It was an eminently defensible site, dominating the crossing from Colchis to Iberia. It was easy to see why the garrison of locals had been replaced with Roman regulars. It was a pity there were not more of them. Freed from the noble shadow of Felix, Ballista slipped easily into the role of senior Roman army commander: touring, inspecting, questioning, speaking words of encouragement. At such times, Hippothous thought, the northerner had an air of competent authority.

At Sarapanis, the expedition was to divide again. Rutilus and Castricius were to travel together as far as Harmozica, the capital of Iberia, to the court of King Hamazasp, the king whose son Ballista had killed. Whenever Hamazasp was mentioned, Hippothous noted, Ballista’s face became hard, closed in: most likely guilt, possibly with an edge of fear. From Hamazasp’s palace at Harmozica, Castricius would journey on to Albania to deal with King Cosis. The latest report placed Cosis at Tzour on the Caspian coast. Undoubtedly, the Albanian king was there keeping an eye on and ingratiating himself with the Sassanid prince Narseh, who was finishing off rebels among the Mardi and the Cadusii just to the south.

As their contubernium was to end, Ballista – subtly, if not indeed unconsciously asserting his primacy – decided that a feast was in order. Hippothous was instructed to produce money from their travelling funds, and soldiers were dispatched to procure the good things necessary. Ballista demanded these include his favourite suckling pig and black pudding, and amphorae of local wine – dozens of amphorae of local wine.

Hippothous came back to life reluctantly. His one servant, Narcissus, was talking to him. Hippothous wished he would stop. The slave continued talking. Hippothous opened his eyes. His head hurt. Narcissus passed him a cup of water. Hippothous sat up and drank it, held it out for more.

The local wine had not come in amphorae but goatskins. It had tasted of goatskins. Hippothous’s mouth still tasted of goatskins. However, he did not feel quite as bad as he had expected. Probably he was still drunk. It meant the full horror of the hangover would overwhelm him later.

‘ Kyrios, the eunuch Mastabates is to talk to you all in an hour.’

In the small headquarters, Ballista, Rutilus and most of the others were waiting. They all looked crapulous. Castricius had not appeared yet.

‘You do not look well, Accensus,’ said Maximus.

Hippothous did not reply.

‘I feel fine.’ Maximus pulled down the neck of his tunic. ‘You should get one of these amethysts. Sure, they are the finest preventative of the effects of over-indulgence. Drink all you want, stay sober, feel good.’

Hippothous noticed that, as well as the gemstone on a thong, the freedman wore a fine golden necklace, Sassanid work. And where did you steal that? he wondered.

‘Cabbage.’ said Rutilus. ‘Fried is best, but boiled will do. Or eat almonds before you start drinking.’

‘Nonsense,’ said Ballista. ‘Old wives’ tales. None of them works, amulets and gemstones least of all. Drink milk first, lines the stomach.’

‘Olive oil, if you are not a barbarian,’ said Rutilus.

Castricius entered to laughter from the rest. The little man looked half-dead.

‘Now we are all here,’ said Mastabates. ‘Before we go our separate ways, I was ordered to remind you all of what information our noble Augustus Gallienus, long may he reign, has received of the three Caucasian kings whose fortifications you will repair and whose allegiance you must secure.’ The young eunuch paused, seemed to swell slightly with pride. ‘I do not think it indiscreet to mention that this order was given to me personally by the Praetorian Prefect Lucius Calpurnius Piso Censorinus. He assured me the information was as accurate as could be found, having been gathered from previous diplomacy, from merchants, and from specially instructed frumentarii .’

‘First, Polemo the king of Suania. He is a man who cares little for either Rome or Persia. He is dominated by two passions – survival, a tricky proposition for a monarch of his race, and the acquisition of as much wealth as Croesus. Polemo’s spirit is dominated by avarice. He taxes heavily those crossing the passes in his territories, and his mountains are said to produce much gold and many gems. Yet none of it is enough to satisfy him. So, he takes gifts from both the imperium and Persia, while keeping faith with neither. Frequently, his men raid the lowlands, as far as the client cities of Rome on the Black Sea coast. Pityous, Sebastopolis, Cygnus – all have suffered. Of course, he always denies responsibility; his warriors act without his permission – which, given his subjects, often may be true.’