A walk along the Celtic fringe

Northumbria is too far from the south for those who don't like travelling and not far or mountainous enough for those who prefer the Scottish Highlands. As it result, it tends to be a little short on holidaymakers. And in the fabulous summer of 1995, our four sons spent several afternoons running over Hadrian's Wall and the surrounding empty landscape. With hand-held computer games abandoned in the car, they revealed themselves as being closet children of the Fifties, entirely able to amuse themselves for hours with hills, stones and water.

Hadrian's Wall is undergoing a long period of renovation and general tarting-up by English Heritage in an effort to conserve and inform. This phenomenal monument, with its extraordinary system of forts, milecastle and turrets, is a gift to anoraks of both varieties - walkers and trainspotters. Subject to gales and generally vile weather, it can be pretty grim. Yet in 1995, Hadrian's Wall was looking positively Mediterranean. If anything, that was a disadvantage, for the great joy of the Wall is the excuse to make lengthy walks from fort to fort admiring the view and deriving the satisfying smugness of the rambler who is enduring wicked conditions.

The most popular site is the fort at Housesteads, largely because of its spectacular location and its state of preservation. Almost all the ramparts and many internal buildings are on show, including the latrine with its Heath-Robinsonesque maze of water-channels. Unfortunately, the consequence is a jammed car park in high summer and a lethal turn off the B6318 for those travelling west.

Be warned - Housesteads is a good walk up a heavy-duty and horizontally challenged track, which means that if you have walking difficulties of any sort, you could have problems. The fort itself is slapped like a wet flannel on the ridge and has very steep gradients in it as well.

Nevertheless, the fort is the best one to visit. If you have time, take a short walk along the Wall to see milecastle number 37 to the west. It's the best preserved and still has part of its gateway arch in place.

The Wall was designed (probably by Emperor Hadrian himself) with a milecastle every mile, and two turrets in between. The plans were changed not long after construction started in around AD122 following the Emperor's visit, and a series of larger cavalry and infantry forts were added, sometimes demolishing a brand-new milecastle or turret to make way. It seems that the forts in which the troops had been stationed were simply too far away.

In fact, Hadrian's Wall is a prototype for the new British Library - a classic example of changes of mind and huge investment followed by abandonment and rebuilding. All of this has made for decades of laborious archaeological investigation in a bid to unravel the sequences. Sadly, the 19th-century excavators were more interested in uncovering walls and buildings, ignoring or destroying later levels as they went.

At Birdoswald (about 16 miles east of Carlisle), the results of recent excavations are now on show. They reveal that the fort here - or bits of it - experienced substantial reconstruction which went on long after the Roman government had abandoned Britain. The military granaries were used as the headquarters of an anonymous community who may have converted one into a kind of chieftain's hall in the fifth century.

But the true history of the excavation of Hadrian's Wall begins at Chesters fort to the east near Hexham. Almost buried in the lush vegetation of the Tyne Valley, Chesters was the home of the 19th-century antiquarian John Clayton. He bought the site, made his home there and dedicated his life to the retrieval of carvings, inscriptions and artefacts from the Wall. They are all on show in his museum, which forms part of the English Heritage site at Chesters.

Unlike most of the Wall area, Chesters is sheltered and has a small cafe. On a grotty day, it's a better prospect than the Cape Horn-style conditions of the high ground. It also has the best-preserved Roman building almost anywhere else in Britain - the extraordinary bathhouse with its high walls, and changing room lockers.

For my money, though, nothing beats Limestone Corner (two and a half miles west of Chesters fort) for a sense of the Wall's timeless presence. Roman ditch-cutters, charged with digging a deep ditch in front of the Wall, struggled manfully on until they got here. First they cut their wedge-holes, then they inserted the wedges, soaked them with water and waited for the rocks to split. Some did, but a lot didn't. So they packed it in and moved on. If you stop and look you can see the abandoned blocks, and the wedge-cuts just as they were nearly 1900 years ago.

For an instant, the passage of time vanishes; you can almost hear the legionnaires saying: "Stuff this for a game of soldiers."

Guy de la Bedoyere is the author of Roman Towns in Britain and Roman Villas and the Countryside published by English Heritage.