Every year Hadrian’s Wall has a hundred thousand visitors, of whom seven thousand walk the 73-mile-long limes, the Roman frontier. The history of this frontier, in the most obscure and unrewarding northwestern province of the Roman Empire, has been under constant investigation for the past 180 years. Its conservation was largely achieved through the passion of just one man (a Newcastle lawyer called John Clayton, 1792–1890), who had the good fortune to have a father called Nathaniel who introduced him at the age of four to his lifelong passion for antiquities.

Every year another book about some historical aspect of “The Wall” is published in Britain, while its influence is embedded in popular culture – witness Game of Thrones and blockbuster films such as Stardust.

By contrast, the easternmost frontier of the Roman Empire is truly the “Hidden Frontier”. Until the publication of this book, no archaeologist had ever worked out the 500-mile route, no historian had written about its forts and no travel writer had marched its length. Yet it guarded some of the richest and most civilised provinces of the entire empire. With the occasional exception (such as Sapor’s invasion of AD256), for 500 years it protected its dazzling, gorgeous cities and ancient temples from the cavalry armies of the Parthian Kings and Sassanid Shahs, as well as the Mountain Lords of Georgia and Armenia in the Caucasus. It is seven times the length of Hadrian’s Wall, and rises to altitudes seven times the height as it climbs across the Taurus Mountains and the Pontic Alps, as well as marching beside the deep, impenetrable gorges of the Upper Euphrates.

As the author writes, “All has remained hidden”, though he is unfailingly courteous to that small but intrepid band of vice-consuls, adventurers and military engineers seconded to the Ottoman Empire who have looked for or stumbled upon the limes: DG Hogarth, VW Yorke, JG Taylor, H von Moltke, Colonel Maunsell, J Brant and Freya Stark.

This all conspires to make Timothy Bruce Mitford’s East of Asia Minor an extraordinary achievement on a host of levels. He is the first man to have consciously walked this entire Roman frontier since the days of Justinian. He is the first historian positively to identify the route that threaded together the line of legionary garrisons and forts. He is also the field worker who has collated the evidence for future generations. I cannot think of another example, in my lifetime, where the different roles of historian, explorer, field surveyor and linguist have been combined so successfully in one man.

What makes it even more poignant is that Mitford, as well as being the first to find and map the frontier, is also the last. For those with the stamina to try to follow his footsteps through the mountains, vast stretches remain, but great sections have already been buried under the rising waters of the irrigation dams that are transforming this region of Turkey, as well as accidentally obliterated by new roadworks, deep ploughing and forestry.

I did at first wonder why the work needed to be spread over two volumes, but once I started delving into his quest, I began to understand Mitford’s desire to record scrupulously. Early on he must have realised that even if he managed to be the first to record the entire frontier, he was certainly going to be the last. The second volume is a meticulous offering to future generations of scholars. We have a citing of every inscription observed (both by himself and cross-referenced with the accounts of earlier travellers), its position, its size and text, often accompanied by a simple, poignant comment – “Lost before 1964”, which proves how invaluable this book is already. We also have the record of every coin find, a hands-on appreciation of the Anatolian landscape in the hinterland of the frontier, a complete list of every known textual reference, a detailed series of modern maps referenced to a topographical index, and an identification of modern Turkish villages with both their Ottoman and ancient identities. The latter were preserved for us by the chance survival of the 225 road routes mapped out for Caracalla (the Antonine Itinerary) and a complete list of officials and military units made for Theodosius in 395 (the Notitia Dignitatum).

The flesh of the work is contained in the 425 pages of volume one. I hope that one day this will be filleted down to become a travel book with a price (down from the current £225) that can put it in the pocket of any reader who enjoys adventure, history and the romance of a quest achieved across a fascinating landscape over a lifetime.

Mitford started on this work as a student, first catching sight of the Euphrates on September 8, 1963. As he self-deprecatingly informs us, the task of exploring this section of the limes took him longer than the legions took to build it. Though he achieved a thesis based on it, his day job was never as a professional classicist like his father (see the Mitford and Bean article in Cornucopia 23) but as a naval officer. Progress was necessarily erratic: the 1960s were highly productive (because the Royal Navy were delighted to encourage a fluent Turkish speaker in their ranks), the 1970s indifferent, but the early 1980s were very good, when his work was supported by the Turkish Navy while he served as British representative at their Ankara headquarters. In the 1990s some resentful bureaucrat awarded Mitford a black mark, which frustrated many a visa application. Fortunately, better-informed Turkish citizens, such as the minister of defence and the chief of staff, trusted Cdr TB Mitford enough to ask him to serve as their interpreter.

However, this enforced longueur brought its own literary benefit, making Mitford an observer not just of the Roman frontier, but of the transformation of Anatolia in the past 50 years. Bear cubs once played under his seat in an antiquated bus on the road to Melitene. He has watched boars as large as small donkeys graze in the evening shadows of a Roman road, and a mile-long line of water buffaloes steaming patiently in the frost before dawn. He has witnessed Roman coins being used as small barter change with which to buy cigarettes, caught the last of the camel caravans festooned with cooking pots and hollow beehive logs, and journeyed on a goatskin raft, lit up by lightning, in a gorge of the Anti-Taurus.

Mitford is an articulate witness to the change of “an ancient and conservative way of life”. He writes of villages “remote, isolated and largely self-sufficient, depending on barter and agriculture, as they had done since the departure of the legion, and radiating a traditional courtesy and hospitality. Drinking water was carried from a spring above the village, ploughing and hauling was by water buffalo, a source of milk, meat, heat and fuel. Processions of wooden-wheeled arabas brought corn from the surrounding fields, and threshing floors were busy with flint-studded sledges and huge wooden winnowing forks.”

We also meet a recognisable cast of villains among the Emperors. Such as the jealous Tiberius, humiliating a proven ally of Rome (loyal old King Archelaus, the last ruler of Cappadocia, descended from both Xerxes and Alexander), just as Nero would destroy Corbulo, an incredibly talented Roman governor. Yet, unlike the British Isles, which only enters written history as a province of the Roman Empire, Rome did not bring civilisation to this part of Turkey. She was just another invader who imposed herself upon Commagene and Cappadocia ten thousand years after the first human-built monuments had arisen on the hills of Anatolia.

If this hidden frontier needs a single identifiable founder for its nomenclature, it has to be Vespasian’s Wall. It was this general who sorted things out in the aftermath of the civil strife of the year of the four emperors. The XII Fulminata legion were dispatched north (away from their disgrace in Judea) to garrison Melitene aided by his own XVI Flavia Firma, a legion of proven worth, who were based on Satala. He had 12,500 legionnaires at work on everything from road- and bridge-building to watching over quarries and making armour, assisted by an even larger number of auxiliary cavalry detachments (who leave a very scant trace of inscriptions) deployed in 29 smaller forts.

From Mitford’s research we can watch the careers of young noblemen from Anatolia, such as Pompilius Piso, serving his apprenticeship as a legate in the legions, before returning to a position of honour in his home city of Ephesus. Or Centurion T Cervonius Lucius, who served for 25 years but died aged 44 in retirement at Ankara. There is tantalisingly little evidence for the soldiers, apart from the roads they carved and the walls they laid. They, and the many Turkish companions of Mitford’s travels (be they gendarmes, scholar schoolmasters, shepherds or distinguished academics), are the heroes of this work.

Now and then, the legions were led by men worthy of their loyalty, such as Alexander the Great’s biographer Arrian, whose understanding of history was informed by his own experience as a Roman frontier general and governor of Cappadocia. In one of Arrian’s letters to the Emperor Hadrian, it is clear that Hadrian had personally inspected the frontier works two years before. The two men had clearly delighted in the fact that a newly surveyed section of the Roman frontier followed the exact march of the Ten Thousand as related in Xenophon’s Anabasis of 370 BC.

On Mitford’s first visit to this place, “one of the most haunting scenes to come down to us from the ancient world”, all he can see in the swirling mist is a sea of clouds a thousand feet below (though six miles later they discover Xenophon’s memorial cairn). Mitford’s guide that day was Celal Yılmaz, whose family had climbed up to this high pass for three months every summer. His father and grandfather told Celal tales of the old days, of the 300-strong camel caravans that used to pass this way, en route from Trabzon to Erzurum, until 1945. The caravans had passed over the spot where Hadrian and Arrian had once themselves stood, marvelling at tales from the past, of Xenophon and his companions greeting the sea (“Thalassa!”) as an old friend.

Thanks to Mitford we now join them. The hidden frontier has been found, and with this book, this achievement, Mitford needs no tombstone.