We arrive in a storm. It is late, the night alive with wind and slanting rain. For the last few miles the narrow lanes, lit by the car’s headlights, have been strewn with a winter confetti of leaves, twigs and branches. In contrast to all this wild movement and scattering through which we’ve been driving, our destination, Llwyn Celyn (meaning holly bush) at the mouth of the Llanthony Valley near Abergavenny, appears as a mass of stillness and solidity, looming dark above us. With no exterior lights and the rain dashing the windscreen, it’s hard to make out much more so, with two sleeping children in the car, I head into the house to find their bedrooms.

As I move through the house turning on lights, the centuries seem to unfold with each illuminated room. Finely carved, pointed ogee arches, heavy flagstone floors, a table etched with ancient initials, decorated and elegantly arching roof timbers, a cavernous bread oven.

In the dining room, a 600-year-old bench is fixed along the far wall. This is, I later learn, where the master of the house once sat at the “high” end of the hall looking through an open fire and a wooden screen towards the “low” end of the buttery and pantry, now a boot room and a study. In a cobbled yard between the kitchen and the cider house (converted into a bedroom and bathroom) I find a huge stone trough receiving a steady flow of the hill’s runoff, its curves, worn over the centuries, making it appear more grown into being than placed.

Facebook Twitter Pinterest A bedroom in the ‘solar’ wing. Photograph: John Miller/Landmark Trust

It’s only with the morning sunlight that the exterior of Llwyn Celyn reveals itself in full, its renovated roof tiled in stone from the nearby Olchon valley and its new coat of white limewash bright against a wide view of fields and hedges on the surrounding low ridges.

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The main building, which has been dated through oxygen isotope research by Swansea University to 1420-21, has the classic floor plan of a vernacular medieval hall house of this area. Attached to it, and making the house an even rarer survivor of the 15th century, is a two-storey solar wing, which housed the lord and lady’s private sleeping chambers. Beyond these living quarters a spread of outbuildings speak not only of the once-high status of this house but also of the self-sufficiency born of this close-knit valley landscape of upland grazing and rich alluvial fields – threshing barn, stables, pigsty, beast house, cidery and kiln house for malt and corn.

Llwyn Celyn was built just five years after Owain Glyndŵr’s rebellion against the English crown (1400-1415), a time when many similar hall houses in Wales were destroyed. Who it was built for isn’t known but given that the land belonged to nearby Llanthony Priory (whose cellar bar is nowadays the perfect end point for walks in the area) it’s possible the first owner of the house was the prior himself.

Facebook Twitter Pinterest The restoration ‘walks a fine line between authenticity and modern comforts’. Photograph: John Miller/Landmark Trust

A succession of farming families inhabited the house after that and in around 1690 one of them, the Watkins family, undertook major improvements, most noticeably inserting a ceiling into the open space of the hall to create a large upper chamber reached by a wooden staircase alongside the substantial chimney stack.

It is to this 17th-century arrangement that the Landmark Trust has, over two years and at a cost of £4.2m, “gently returned” Llwyn Celyn, discovering bread ovens, fireplaces, windows, doorways and even a pair of shoes (to ward off witchcraft) behind layers of 19th- and 20th-century plasterboard. The restoration walks a fine line between authenticity and modern comforts (most rooms have underfloor heating, the bathrooms rose head showers and in the living room a wood-burning stove, not an open fire, now throws its flame-light over the rafters) but it is also, as one carpenter who worked on the house put it, “honest”. New inserts and graftings of wood have been left a lighter shade than the original carpentry, so the building wears the patchwork of its recovery on its sleeve.

Facebook Twitter Pinterest The entrance hall at Llwyn Celyn. Photograph: John Miller/Landmark Trust

Lying on a sofa in Llwyn Celyn looking over this play of light and dark in the structure above, it’s impossible not to feel, at a visceral level, the decay from which the house has been saved. It’s a sensation amplified for me by the fact that for many years when I was younger, I only ever knew Llwyn Celyn – as I walked, drove or rode past it – as a rough canvas of emergency scaffolding and sheeting. What exactly lay under those coverings I could only ever imagine, so to find myself now eating, sleeping and playing with my children within its walls is an experience both strange and comforting.

Just as Llwyn Celyn remained unknown to me when I was younger, so did its immediate surroundings. The Llanthony Valley, a few miles up the road, is a walkers’ and runners’ playground, with numerous paths and trails between the Hatteral Ridge and Offa’s Dyke to the east, and the ridge of Grwyne Fawr to the west. It’s also a place rich in history, from the intricate 15th-century rood screen in the church at Patrishow to the chapel and monastery at Capel-y-Ffin, once the home of sculptor and typographer Eric Gill and, briefly, the artist and writer David Jones. The ridges of the upper valley, therefore, were always my destination rather than the gentler undulations at its mouth. With this in mind we choose to walk from Llwyn Celyn itself, taking a circular route through the mature beech, oak and sycamore woodland of Coed y Cerrig then rising up around Twyn y Gaer, an iron-age hill fort, before dropping back down to the Queen’s Head near Cwmyoy for lunch.

Facebook Twitter Pinterest The kitchen. Photograph: John Miller/Landmark Trust

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The woods feel remarkably in tune with the atmosphere and age of the house from which we began our walk, and as we climb the slope under their moss-covered branches, lines of a medieval poem by Llwelyn ap y Moel, To the Greenock Woods, come to mind:

Faultless nurture, it’s been good

To have you as my safeguard,

Sweet close and veil for refuge,

Strong and swiftly sheltering hedge,

beneath me level greensward,

Green, kind earth, gem of a lord,

Trusses of sweet leaves crowded

Like a dark tent overhead

Trans. Tony Conran

During our stay at Llwyn Celyn, I was struck by how all of us, parents and children alike, kept touching the house as we moved through it – window sills, beams, doors – as if to attempt to read them and their centuries of witness. I don’t know how successfully we did it, but certainly on leaving my wife and I felt an odd sense of ownership, of a quality we’ve never known with other holiday rentals. Llwyn Celyn had, undoubtedly, over our brief time there, contributed to our lives.

• Accommodation was provided by the Landmark Trust. Llwyn Celyn costs £909 four nights and sleeps eight

Owen Sheers is the author of To Provide All People: A Poem in the Voice of the NHS (Faber, £12.99), available at The Guardian Bookshop for £11.17