That the gods had granted them the rule of the entire globe was a claim forever being trumpeted by the Romans. "Imperium sine fine", they termed their empire: "dominion without limit." The truth, of course, was somewhat different - as the Romans themselves were reluctantly aware. The haughtiness of their pretensions, as is so often the case with successful imperial peoples, was combined with a hard-edged realism. This was why, in the cause of securing the continued viability of their empire, they were prepared to accept that it might, after all, have limits. The wild and profitless extremities of the world could legitimately be left to barbarians. The result, snaking for thousands upon thousands of miles across three continents, was the ancient world's most astounding frontier.

Indeed, that the empire is long gone, and the frontier with it, paradoxically serves to make the immensity of territory that once acknowledged Roman mastery appear all the more stupefying. Perhaps only those prepared to tramp the moors of Northumbria, the sands of Arabia and everything in between can truly hope to get their heads around its sheer scale. That is why the idea which underpins Philip Parker's new book is such an intriguing one: to trace the entire length of what the Romans themselves termed the "limes", the frontier zone of their empire. The result was a journey epic enough to satisfy even a Virgil. As Parker sums it up, with justifiable pride, "I have encountered more than five centuries of Roman history, in some 21 modern countries, covering a range of climactic variations from a snowstorm in Switzerland to a sandstorm at 45 Centigrade in Egypt's Dakhleh Oasis, and have covered more than 20,000 kilometres on the ground."

Yet his book is far from being a conventional travelogue. Once the introduction is done, the first person barely intrudes. Neither a work of history, nor a scholarly gazetteer, nor a guide, but rather a blend of all three, The Empire Stops Here is a book in which weather-beaten masonry serves to crowd out human beings, and in which the people who most truly come alive are those who have been dead for 2,000-odd years. The effect, in the opening chapters, can often be alienating - and all the more so for the fact that Parker has chosen to open his odyssey with Hadrian's Wall, long a staple of less serious travel writers. When he comes across an advert posted by "Jefficus", a Roman re-enactor, for instance, it echoes the very similar serendipities that so delighted Hunter Davies in his warm and witty book, A Walk Along the Wall. Unlike Davies, however, Parker has no interest in dwelling on whimsy. The scale of his ambitions do not permit it. Germania and Pannonia, Cappadocia and Egypt: all are awaiting him. Jefficus is accordingly dismissed in a sentence.

Further re-enactors are encountered in the course of Parker's travels - some Dutch office-workers practising archery, a class of Austrian schoolchildren dressed up as gladiators - and again, he does nothing so vulgar as actually to engage with them. By the time we have followed him to Austria, however, we are starting to wake up to the full originality of his project; and Parker himself, perhaps, has grown more comfortable with it, and more self-assured. Increasingly, like early-morning mist veiling a mountain range, his Brysonesque stabs at observational humour fade away, to reveal in all its magnificence a quite breathtaking and eccentric edifice of scholarship. Parker's true models, it turns out, are not the modern generation of travel writers at all, but rather the ancient geographers, scholars such as Strabo and Diodorus Siculus, who thought nothing of using their travels as pegs on which to hang entire histories of the world.

Certainly, barely a place is visited but it affords Parker an opportunity to examine some fascinating aspect of the Roman past, be it the chronicle of a campaign, the character sketch of a Caesar or an analysis of some aspect of daily life, from Mithraism to gladiators to baths. The result is a portrait of the empire very similar to some of the more impressive monuments that Parker visits, which, reconstructed by archaeologists out of assorted fragments, serve to hint at the vanished whole. Indeed, it is a curious effect of his method that even the most brutal intrusions of the modern on the ancient past - whether it's the construction of a noodle bar over a hypocaust in Vienna or of a prison across the corner of a legionary camp in Algeria - seem to diminish not antiquity, but rather the present. Perhaps that is why the book's most haunting and atmospheric section should be the one that covers the eastern frontier, where the mute ruins of great cities set among deserts, or else "barred off by swathes of barbed wire", stand as the most eloquent witnesses of all to the empire's fall.

Granted, the news that Rome has gone the way of Nineveh and Tyre hardly comes as a bombshell. The inherent mutability of things has been standard fare in travel writing, from Pausanias to Sebald. Nevertheless, the disjunction between antiquity and the present is very far from being Parker's only theme. Most of what he sees inevitably bears testimony to the ruin of Roman greatness, but he is also fascinated by the enduring trace elements that it has left in the world of today. For every Petra or Palmyra, cities so abandoned as to appear half as old as time, there is a Vienna or a Budapest, contemporary metropolises that "owe their existence to their origins as Roman legionary camps, sites whose location was so strategically placed that their associated civil settlements survived the catastrophe of the empire's fall in the west, continued to grow and eventually became capital cities."

In truth, historians tend to dispute the degree to which urbanism in western Europe actually owed anything to Roman foundations: for even when European cities stand on the physical sites of ancient predecessors, there is a sense in which it is only the continuity of their location which has survived from antiquity. Yet it is indisputably moving, for all that, to recognise the common residue of inheritance that so many different countries, so many different regions and so many different peoples share. The same Constantine, for instance, who was hailed as emperor in York, who founded the city that would become Istanbul, and who built the Church of the Holy Sepulchre in Jerusalem, has also, so Parker informs us, bequeathed his name to the airport that serves his birthplace: what is now the city of Nis in Serbia. "It is," we are informed, "the only such facility anywhere to have been named after a Roman emperor": grist to the mill of anyone setting a pub quiz.

Unlike Shelley's traveller from an antique land, Parker does not write in scorn of the colossal wreck that he has witnessed, but rather in praise of it. His travels, he confesses, prompted in him two emotions: "wonder that after close to 2,000 years so much can survive, and sadness that for all our sophistication, we are unlikely ever again to create something so enduring." At least he can console himself with the reflection that, with this extraordinary book, he has raised a monument all of his own.

 Tom Holland's books include Millennium: The End of the World and the Forging of Christendom (Abacus). To order The Empire Stops Here for £23 with free UK p&p call Guardian book service on 0330 333 6846. Philip Parker talks to Claire Armitstead at guardian.co.uk/books