Wherever you walk in Wales, it turns out that you’re tracking some ancient, mythic storyline or other – and nowhere more so than at Tomen y Mur (“the mound on the wall”), a Roman fort and amphitheatre in Snowdonia national park, which sits beside a crossing of four Roman roads.

Tomen y Mur, surprisingly unsignposted in spite of its significance, sits on the lower slopes of Mynydd Maentwrog, not far from the A470 to the north-east of Llyn Trawsfynydd reservoir. The fort, now a scheduled monument, was constructed under governor Gnaeus Julius Agricola in 78AD, as part of his campaign to control the native Ordovices, a Celtic tribe who resisted Roman rule with some enthusiasm. Agricola’s response was a brutal campaign which, according to Roman chronicler (and Agricola’s son-in-law) Tacitus, almost wiped out the tribe.

The Roman remains at Tomen y Mur are extensive, and various earthworks outside the fort are believed to be a parade ground, a bathhouse, a mansio (a house for visiting officials), burial mounds, a temple and a rare, small military amphitheatre. Abandoned in 140AD, the site was reoccupied a millennium later, and a castle mound built inside the original walls. It was probably constructed by the Normans to counter the Welsh insurgency of 1095.

It’s a lovely site, surrounded by fields dotted with the inevitable sheep. There are panoramic views to the west across Llyn Trawsfynydd, a large reservoir on whose north-eastern shore squats a rather less inevitable, decommissioned nuclear power station. The mountains of Snowdonia glower from all directions – the Moelwynion to the north, the Arenigau range stretching east and south-east, and the Rhinogydd to the south and west.

Here history and mythology coincide. Tomen y Mur is identified with the legendary palace of Mur Castell in The Mabinogion – a masterpiece of medieval Welsh literature, replete with magical tales reflecting an ancient oral tradition. Its fourth “branch” tells of the prince Lleu Llaw Gyffes (“the fair-haired one with the skilful hand”) who lived here with his wife Blodeuwedd. Blodeuwedd (“flower-face”) had been created for him from the blossoms of oak, broom and meadowsweet after his mother, Arianrhod, cursed him and declared that he should never have a human wife.

But Blodeuwedd had her own idea about what she wanted from life, and it didn’t involve Lleu. So she conspired with her lover, Gronw, to kill him and, according to the story, the two ruled together from Tomen y Mur after Lleu’s death. But unfortunately for Blodeuwedd, Lleu wasn’t altogether dead – he had simply shapeshifted into an eagle after Gronw’s attack, and was eventually transformed back into human form by his uncle, the great magician Gwydion. Lleu killed Gronw, and Blodeuwedd was turned into an owl as punishment.

“You will not dare to show your face ever again in the light of day,” Gwydion told her, “and that will be because of enmity between you and all other birds.”

It’s not just at Tomen y Mur that we can walk these mythic tracks: the surrounding landscape is also steeped in this story. The stone with which Gronw tried to protect himself when Lleu came after him was said by the narrator of The Mabinogion to be “on the bank of the River Cynfael in Ardudwy”, with a hole in the centre where Lleu’s spear penetrated it, so killing Gronw outright. In the early 1990s, a stone slab with precisely such a hole in it was found hidden in the corner of a field on the banks of Afon Bryn Saeth, a small stream that runs into the Cynfael. The stone is now on a farmstead known as Bryn Saeth, or Hill of the Arrow; it is locally known as Llech Ronw – “Gronw’s Slate”.

History fades, but the myths embedded in the land don’t die. The soldiers at Tomen y Mur may now have been entirely replaced by sheep, but the lonely hooting of a short-eared owl on a Snowdonia hillside at dusk is a reminder that the old stories are stalking us still.

• Sharon Blackie’s collection of short stories, Foxfire, Wolfskin and other Stories of Shapeshifting Women, is out now (£14.99, September Publishing); to order a copy for £13.19 visit the Guardian Bookshop

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