Hadrian, now, he was familiar: Roman emperor, liked walls, loathed Scots. But who on earth was Offa? I admit I knew little of the King of Mercia until I took to the National Trail – Offa's Dyke Path – which bears his name and wends 177 miles from Chepstow in south Wales to Prestatyn on the north coast, crossing the English-Welsh border more than 20 times in the process.
Whatever your knowledge of Offa, it's an enticing walk, embracing river valleys, abbeys, farmland and bleak ridgetops that are tough to access any way except on foot. This year the trail turned 40, which is what brought it to my notice, but the boundary it follows is significantly more venerable.
Offa (I now know) became King of Mercia in the year 757; he expanded his territory across much of England but failed to conquer the Welsh. So he built a barrier to keep them contained – not from stone, Hadrian-style, but mud: a deep ditch backed by a mound of earth. A muddy ditch is inevitably a less permanent structure than a wall, which perhaps explains why it doesn't have Hadrian-like status. But it has the same border-defining mystique. And there's something appealing about tracing a national boundary, even if no one wants to see your passport.
The four days at my disposal were not enough to walk the entire trail, which takes 12 to 14 days, but it was certainly enough time to allow a fine long-weekend yomp north from Chepstow Castle's 11th-century fortifications to the biblio-heaven of Hay-on-Wye (where, incidentally, books will be sidelined by boots on Friday when the town's first walking festival gets underway). I had approximately 55 miles in which to get acquainted with Offa, his legacy and some of the sights en route.
Sunshine glittered on the Severn Estuary as I left Chepstow; it felt like being beside the sea. Soon, though, I left the views, delving into forest that felt old as Offa to meet the Dyke itself. The aged earthwork is mostly gone, but stretches of the National Trail trace clearly discernible chunks of the ancient boundary. I followed some of it here: leaf-littered and tree-shrouded, but a dyke nonetheless. It felt primeval; the sheer weight of years seeped into my boots, soundtracking a detour down tree-root ladders, through woodland, to visit the hulking remains of Tintern Abbey.
A newcomer by Offa's Dyke standards, Tintern was built in 1131 by Cistercians who abstained from just about everything. I abstained from nothing, consuming ice cream and shandy in the pub opposite the medieval abbey's vast, ruined vaulting. What a spot: the monks chose this hideaway, deep in the Wye Valley, to escape from the unwanted attentions of men; centuries earlier Offa, too, had used the natural (and spectacular) rise of the valley to keep the unwanted away.
A sense of vintage seemed to be order of the day – after wonderful walking through hilly, flower-flecked country, I detoured from the path for a night's rest in St Briavels. On the Gloucestershire side of the border, the village is home to a suitably old beamed pub, and an even older red sandstone castle, its Norman bailey now an unusually historic hostel.
No time for regal lingering though: the next day I was up with the sun to rejoin the path, bidding the local deer good morning on my way. I had a long stretch ahead, my aim the hamlet of Llangattock Lingoed, via treetops rattling with woodpeckers, the relative bustle of Monmouth (and the big blueberry buns of its Wriggles bakery), rolling hillsides, and the turrets of White Castle. (You know you're in frontier country when such bastions crop up more often than houses.)
By the time Llangattock's whitewashed medieval church hove into view, I was more than ready for a Radox bath and a pint of real ale at the adjacent cosy inn, but utterly in love with this undulating, unpeopled borderland. If day two had offered sheep-filled fields and fairytale woods, day three was a different proposition. No more valley-dipping: now I was climbing on the back of the Black Mountains, ascending to 1,600ft and feeling like the ruler of both sides of the border in the process.
Clement weather – disappointingly calm for the paragliders waiting hopefully on the ridge's flanks – allowed me to survey with ease the drama of the Brecon Beacons to the west, the more bucolic ripples of Herefordshire to the east. From my lookout I could see the romantic ruins of Llanthony, my home for the night, long before I reached them. If it was a precipitous scramble down to the secretive Vale of Ewyas, in which the 12th-century Augustinian priory sits, it was possibly even hairier ascending the 60-odd tight spiral steps winding up the Priory Hotel's tower.
My room, tucked within the ancient building, wasn't plush, though it somehow held a four-poster. It was also magical, its tiny window looking down into the grounds, atmospherically deserted once the day-trippers had gone, with the Black Mountains framed through the old stone arches. It was back up to those mountains the next morning, a calf-testing climb to a land transformed overnight: the views were gone, obscured by a thick mist that erased centuries and left me tramping, unshielded from the elements, in a netherworld of heather, bracken and howling wind.
Thankfully, the path was well marked, though while I didn't fancy getting lost, the penalties are far less severe than in times gone by. In the days of Offa, I'd been told, the Welsh hung any Englishmen found on their side of the divide; the English relieved trespassing Welshmen of their ears. It was a wild bit of walking – drizzly but dramatic.
As the ridge dropped, spongy heath led to fields filled with oddly belligerent sheep. The landscape softened further, the sun peeped through, and by the time I entered the civilised environs of Hay-on-Wye, the morning's bleakness seemed to belong to a different dimension. I cruised Hay's second-hand bookshops, searching for tomes on Welsh folklore. I cappuccino-ed in hip arts spaces and feasted in good pubs, raising a glass to Offa's obstruction as I did so. Because though his Dyke was designed to keep people out, the path that now follows it through this fascinating borderland offers hikers a rather warmer welcome.
St Briavels Youth Hostel (0845 371 9042; www.yha.org.uk) is housed in a Norman castle. Dorm beds from £14.40.
Hunters Moon Inn, Llangattock Lingoed (01873 821499; www.hunters-moon-inn.co.uk) is a friendly pub. Doubles from £65, with breakfast.
Llanthony Priory (01873 890487; www.llanthonyprioryhotel.co.uk) offers accommodation in the 12th-century ruin. Doubles from £80, with breakfast.
Tinto House, Hay (01497 821556; www.tinto-house.co.uk) is a lovely B&B. Doubles from £85.
Offa's Dyke Path: www.nationaltrail.co.uk/offasdyke. "Offa's Dyke Path" (£11.99; Trailblazer) is a useful guide.
The Hay-on-Wye Walking Festival starts on Friday ( www.haywalkingfestival.co.uk).