Perched on a hillside looking out across an arid valley of rocks and pomegranate groves, the mud-brick walls of Kharanaq have changed very little in the past few hundred years. What has changed though, is what lies within them. An 80 km drive from the historic city of Yazd – right in the heart of Iran – this once thriving citadel on the desert’s edge now lies abandoned; its arches and domes returning slowly to dust under the timeless gaze of the mountains.

I took a trip around Iran with my friend Yomadic, and our small group arrives at Kharanaq by minibus, parking beside the old caravanserai. These traditional roadside inns are a staple of travel in Iran, and this one features several cool stone courtyards between wooden walls. There was a caravanserai here as early as the pre-Islamic Sassanid Empire (224-651 AD), apparently, though the current building dates to the 17th century. Later they’ll serve us lunch here – grilled meats, aubergine paste and sweet-cooked rice with saffron – but first, we are set loose in the ghost town.

You could almost drive past Kharanaq without noticing it. The road bisects the village, where newer buildings surround the caravanserai on one side, while opposite, a plain, high wall masks the crumbling history beyond. Old men sit in patches of shade along the roadside, drinking tea, while a young boy tugs at the rope around a donkey’s neck, eventually wrestling the beast into submission then mounting up, to trot up and down the cobbles past the newly-arrived tourists. We duck through an arch in the wall whereupon the old village reveals itself. Even at first glance it has all the promise of a maze: the tall buildings offering a choice between three branching alleyways, each of which leads to more multiple-choice junctions. I pick a random direction, then another, and just keep walking. It doesn’t take long before I’m completely disorientated, out of sight and sound of the group I came with.

Kharanaq is much larger than it appears from the road. This village – or walled citadel, more like – spills away down the hilltop, its buildings several floors high and connected through superimposed maps of pitch-black ground level tunnels, and dusty rooftop walkways. The buildings bulge into the street, organic curves resulting from years of mud and plaster repairs, slapped onto the dry walls one handful after another. In places, they have crumbled: walls turning to yellow powder, to reveal tufts of preserved straw fibre who knows how old.

As in Yazd, water was provided to Kharanaq via a Qanat system: a kind of underground aqueduct built into a slope that brought water from a main well to a series of vertical access shafts throughout the village. Here and there, tucked into corners between high mud walls, are half-covered pits, now clogged with debris, which appear to be all that remains of this once-revolutionary water system. From the narrow alleys, I look inside the ground level spaces of empty homes. Traditionally, the people here kept sheep or other livestock below, and lived with their families on the upper floors. Security and central heating combined.

Sometimes stairwells lead up inside, but more often the human spaces feel connected as if by a different network, accessed by uneven stairs, that curve around the outside of bulbous walls, or follow precarious paths over the brickwork domes and arches. Cresting one of those arches, I reach a viewpoint: the village rooftops drop away to reveal the valley beyond. Hard, dry mountains form a sharply contoured perimeter around the fields beneath Kharanaq. Crops are marked out in tidy rows of yellow and olive green, while in the distance, tucked in amongst the bases of mountains, rises the faded pastel-blue dome of a mosque. It is a shrine – apparently containing the hand of Imam Reza, one of the twelve divinely ordained Imams of Shia Islam.

I spot members of my group on other rooftops, and I notice that there are Iranian tourists here too. Families walk their children through the tight passageways, or clamber up to rooftops for this view. Meanwhile clouds roll over the valley, casting the distant dome in alternating pools of light and shadow, and I watch for a while longer before making my way back down into the labyrinthine streets of Kharanaq.

Some Iranians claim that Kharanaq is more than four thousand years old – counting from the first evidence of human settlement in this area. However architectural historians make a safer claim of one thousand years. The location of the nearby Chak Chak fire temple – one of Iran’s most famous Zoroastrian temples – would suggest that in pre-Islamic times this was a Zoroastrian settlement, existing perhaps as a stopping place for merchants and caravans on the pilgrimage route. Some of the 80 houses that compose the citadel have been dated to the 14th and 15th centuries… other elements suggest pre-Islamic architecture, in the Seljuk style associated with the 10th and 11th centuries. However all structures share the same basic principle, having been designed to withstand the severe climate of the region, while benefitting from a fortified hilltop position to defend against enemy attacks.

Though it may look like a long-lost ancient city, Kharanaq was still inhabited as recently as the 1960s. Its residents abandoned their homes for more stable, accessible, modern houses nearby. The government offered bonuses of free land and electricity to those relocating to this ‘New Kharanaq’: a simple settlement built on a grid-like structure near the old citadel, which, as of 2006, was home to 133 families, or something like 350 people in total. The mud-brick houses of the old citadel meanwhile, deprived of regular maintenance, grew brittle in the sun and quickly began to disintegrate.

The further I get from the entrance, the worse these buildings look. Rooftops are split by cracks and chasms, while bedrooms spill through floors into sheepfolds below; in some places, sunlight pierces down through three floors of ruined architecture. At the far end of the village, a few buildings have completely collapsed under their own weight. One house, tucked back a block from the outer wall, nevertheless now enjoys an unobstructed view of the valley outside; its several neighbouring houses having already crumbled to dust. I follow a path above the debris, through one home and onto the mud-and-straw rooftop of the next, following the well-worn pathway… when suddenly my foot goes straight through it.

There is an uncomfortable fragility to Kharanaq today, and it is hard not to question the effect that modern visitors are having on it. Iranian tour guides encourage visitors to dive in and explore… nothing here is off-limits, and domestic tourists can also be seen scaling these crumbling houses like mountain goats. But I am left with the feeling that this experience can only be had so many times. Nearby Yazd is already recognised as a UNESCO World Heritage Site. Supposedly Iran wants the same for Kharanaq, but no such protection has been forthcoming, and without protection, it makes no difference whether tourists come here or not: Kharanaq is not long for this world.

Perhaps I’m still feeling guilty about my contribution to the decline, because when I reach the far end of the village I come back not through the main paths, over the rooftops and the split-level walkways, but rather, around the outside of the citadel walls, through the sandy scrubland. Originally topped with six defensive watchtowers, now these walls are mostly a symbolic gesture; a decorative perimeter. Seen from this angle the village is more imposing, its organic curves and inviting corners replaced by a dense and derelict stack of multilevel housing. From outside the walls it looks like a dead husk… but from down the valley there come simultaneous sounds of life. A procession of villagers are making their way down from New Kharanaq, over the old stone bridge, to the mosque for prayers. Black chadors whip and ripple in the desert winds.

From the hillside, a track cuts back into the warren-like village, squeezing in between two buildings shaped like cliff faces. Nearby a wooden door is set into a rough edifice of rock – this one a natural cliff, I realise, almost camouflaged against the buildings, but hollowed out and put to use all the same.

The path leads me back up towards the archway where we entered Kharanaq, and past some of the newer buildings at the roadside end of the village. Here, the mosque and minaret were newer additions, added in the 17th century, like the caravanserai.

Close by, I follow some winding steps down into shadow and suddenly find myself in a bathhouse. Compared to the mud-brick walls of the old village streets, this place is a striking contrast. The tiles look relatively new, as if renovating this bathhouse was one of the last community projects undertaken by the residents of Kharanaq before they left in the 1960s. Inside, voices echo off the tiles in a language that isn’t English, and isn’t Farsi either. I turn to leave, and find the stairs blocked by a crowd of Chinese tourists who’ve just arrived with their own tour bus.

When we drive away from Kharanaq, a storm is brewing above the distant mountains. Its arches rise in silhouette against the clouds and figures move about on top, tourists posing for photos on these thousand-year-old structures, and then the road turns and the citadel is lost from sight. Kharanaq did not survive all this time by being fragile, but rather, these buildings were organic like a bee nest, constantly growing, evolving, repairing. Kharanaq shared a symbiotic relationship with the people it sheltered – and without them, it is simply unsustainable. It remains to be seen whether or not Iran is able to preserve this site before too much of it is lost; but even preserved, even protected as a UNESCO site, there is a sense here that without the community who made this citadel their home, Kharanaq will never again be anything more than a ghost town.