IMAGINE digging a shallow trench for your potatoes. Now deepen it to 10ft, bank up the sides by the same amount, then using only a primitive wooden tool continue it lengthways for, say, 180 miles through some serious hill country.

Welcome to Offa’s Dyke.

What inspired the all-powerful King Offa, 8th-century ruler of Mercia, to dig a vast ditch the length of his kingdom will never be known for certain. Today the great linear earthwork forms the basis for a superb 177-mile waymarked footpath that runs the length of the Welsh border – from the Severn Estuary near Chepstow to the Irish Sea at Prestatyn. More than twice the length of Hadrian’s Wall, it is Britain’s longest ancient monument.

We wanted to experience the best of Offa in a long weekend, so my wife and I began our three-day, 41-mile walk at the traditional border town of Kington, north-west of Hereford, from where the trail winds northwards through a series of remote and undulating valleys.

Although the slopes soon got us puffing, the sight of the dyke rolling away ahead was thrilling. In some places the dyke is merely a shallow depression, but elsewhere the height from the ditch to the top of the bank is as much as 20ft.

Because of erosion, the Offa’s Dyke National Trail usually runs alongside rather than on the actual earthwork, but there’s plenty of scope for you to explore it at close hand and ponder the mysteries of the Dark Ages.

Come 7pm on our first day, the only mystery for us was deciding what to choose from the pudding menu in the George & Dragon Inn at Knighton, our base for that evening. I reasoned that a 14-mile hill walk justified a super-sized sticky toffee pudding.

Next morning we set off early, climbing above the River Teme. Red kites and buzzards soared overhead in the early morning sunshine and there were fantastic views over the roofs of Shropshire and Powys. That afternoon, as we tramped through dense woodland and open meadows, passing only the odd tractor, it felt like we were the only people for miles.

Day two ended with a short diversion to Clun, where we checked into a restored water mill that is now home to a youth hostel. Pleasantly weary, we rested our aching limbs at the White Horse Inn on The Square, which serves
hearty home-cooked dishes and a fine selection of local real ales.

We resumed northwards through the Shropshire Hills on our final day. For the last few miles we walked a Roman road that scales Beacon Ring Iron Age hillfort above Welshpool, with its sweeping views of the Severn Valley. I later learnt the trees were planted to mark the coronation of Queen Elizabeth II in 1953.

The questions surrounding the remarkable creation of Offa’s Dyke, however, were still very much unanswered. Perhaps some mysteries are best left unexplained. 

GETTING THERE: Maps: Ordnance Survey Explorer 201 Knighton & Presteigne, 216 Welshpool & Montgomery.
The George & Dragon, Knighton (01547 528532) offers doubles from £27.50 per night (two sharing), B&B.
YHA Clun Mill (0870 770 5766/www.yha.org.uk) offers accommodation from £11.95 per adult, £8.95 per child (under 18).
Visit Wales: 08701 211251/www.visitwales.co.uk.