The Wall: Rome's Greatest Frontier, By Alistair Moffat

Hadrian's Wall can no more be seen from space than the Great Wall of China, but it remains one of the greatest feats of the Roman era. Running 73 miles from sea to shining sea, and then as much as another 50 south down the Cumbrian coast – a much-neglected detail – it was a stony, unflinching statement of Roman power and prestige.

Its instigator, the Hellenophile Hadrian, travelled incessantly, loved boys as well as women, and like all the best generals and emperors – Scipio, Caesar, Vespasian – lived as hard as his men in the field, camping rough and drinking the same thin, sour wine as his soldiers. The monument he built – or rather, his II, VI and XX legions – certainly displays all the "unbending rigidity of thought" that we associate with the Romans.

But it is much misunderstood. It wasn't built to "keep out the Scots", or the Picts. It was far too narrow to be a fighting rampart, and there were no abutments for enfilading fire against attackers. Rather, the Wall was an implacable border marker, a customs collection point, and a serious impediment to any Northern tribe planning to raid south: not a full stop, but certainly a mighty comma.

The Romans liked things in black and white, Romans and barbarians neatly divided – except it's never that black and white. As Moffat points out, it's we, not they, who show the "unbending rigidity of thought" when it comes to sex and race. They had no concept of "straight" or "gay", for instance, and obsessing about ethnicity would have puzzled them too. Being a Roman citizen was all about cultural superiority, literacy and the rule of law, and skin colour was irrelevant. From a chance find at Maryport we know that commanders of the I Cohort Hispaniae were variously from Verona, Nîmes, Solva (today in Austria), and Tunisia. From the viewpoint of a Victorian race theorist, the Romans, from the Emperor down, were the most appalling mix of half castes, mongrels and octaroons.

They were tough as old leather, though. Roman infantry, Marius's Mules, could march 20 miles in five hours with full kit, weighing well over 100lbs, a level of fitness equalled by only the most highly trained soldiers today.

One of the real joys of Moffat's account is his sensitivity to etymology, which can be a luminous road into the past. The name of the great Pictish chieftain Calgacus means simply "The Swordsman", while tribal names are clearly animal-totemic: the Carvetti of the Eden Valley were the Deer People, the Lugi of Sutherland the Raven People, and the Venicones of Fife, the Kindred Hounds. The Wall forts are enchanting too when translated: Aballava (Burgh-by-Sands) means Appletree Fort, Bibra (Beckfoot), Beaver Fort, and Gabrosentum (Moresby), Goat's Path Fort. Schiehallion means "the Magic Mountain of the Caledonians". I wish I'd known that last January, when I climbed it on my own, in heavy snow, and nearly died.

Moffat sometimes tells us things that are surely universally known already, such as the fact that the Romans ate dinner lying on couches. But you can forgive him much for his wonderfully entertaining digressions, sometimes quite a long way from the Wall itself. He can't resist a fact box, giving us the names of some of the filles de joie known to have worked in Pompeii: Culibona (Lovely Bum), and the obliging Panta (Everything), while the poor girls had to contend with regular customers called things like Skordopordonikos (Garlic Farter). I never knew that the little village of Pitigliano in Tuscany still has its own dictionary, its dialect being quite incomprehensible even to fluent Italian speakers.

Hadrian had a sad end, living out his last lonely, dropsical days in the gardens of Tivoli, grieving for his beloved Antinous, before dying "hated by all", one chronicle says, having made so many enemies that the Senate refused him his routine post-mortem deification. But much of his Wall remains impressive to this day, and a reader-friendly chapter at the end recommends not only some good walks, but even a pub for lunch.