It has been damned as the world’s worst ever restoration, yet another national embarrassment to add to Spain’s inglorious track record of botched conservation projects. The quaintly crumbling ruins of the ninth-century Matrera castle in Cádiz province have been invaded by a white concrete hulk, the precious Moorish stone walls reduced to a thin rind of history, stuck on the front of a big blank box. It is one of the most extreme facadectomies of modern times.

The project has been the subject of derision and disbelief across social media, decried as “absolutely terrible” by national heritage body Hispania Nostra. “No words are needed,” they added, “you just need to look at the photographs.” But look at the photographs and you may well be witnessing a work of accidental genius.

Until local architect Carlos Quevedo got his hands on this protected national monument, in Villamartín, it was just another ruined Andalusian fortress – indistinguishable from those topping practically every hill in the region. Now it has been mutilated into a startling Frankenstein bunker, it has become an international celebrity.

It can also join Spain’s illustrious history of inadvertent masterpieces. When 80-year-old Cecilia Giménez got to work with her pot of paints, the fresco of Jesus in the church of Santuario de la Misericordia, in the small town of Borja, was simply another fresco of Jesus in an unremarkable church. Since her inspired creation of the smeary-faced ape-Christ – instant meme-fodder around the world in 2012 – the village has become a place of pilgrimage, seeing thousands of visitors, a booming novelty T-shirt business and even inspiring a comic opera.

Whether Quevedo’s neo-brutalist insertion has quite the same comic value as the smeary Jesus of Borja remains to be seen, but the architect is adamant his work is in keeping with the original building’s spirit. It is certainly more forbidding than its original creator, the fearsome Christian anti-Umayyad leader, Umar ibn Hafsun, could ever have hoped.

There were three basic aims, Quevedo told the Guardian. “To structurally consolidate those elements that were at risk; to differentiate new additions from the original structure – thus avoiding the imitative reconstructions that are prohibited by law; and to recover the volume, texture and tonality that the tower would originally have had.”

Squint a bit, and you can sort of see what he was trying to do. His approach follows a recent fashion for restoring ruins with blank additions, rebuilding the general volume of what the original structure might have been, but without any of the detail or decoration. The spirit of the original is revived, in its mineral bulk and heft, so the argument goes, but without pretending to construct an exact replica or resorting to shallow pastiche.

Perhaps Quevedo had seen David Chipperfield’s model of his proposal for the 15th-century Castello Sforzesco in Milan, in which he planned to fill in the ruined battlements with a solid mass, devoid of texture or decoration. But maybe he didn’t realise that the stark white blocks in the model were intended to be built in brick and stone, of a tone that chimed with the original – not rendered in white concrete, as he has chosen to do.

The Stirling prize-winning Astley Castle, by Witherford Watson Mann, followed a similar logic, inserting sharp blank walls of brick into the burnt-out ruins of an old manor house. Some heritage purists once again decried what they saw as a blunt and disrespectful intervention, but visit the building and you will find a finely wrought collage of old and new, stitched together with unparalleled sensitivity and care.

One of the precedents for Astley was another project in Spain that was met with equal controversy to the Matrera castle when it was unveiled. The Roman theatre in Sagunto, near Valencia – one of the first structures to be declared a national monument, in 1896 – was radically overhauled in the early 1990s by Italian architects Giorgio Grassi and Manuel Portaceli. They smothered the crumbling stone steps with bright white limestone seats and erected a 25-metre high stage front in brick and stone, ignoring a “stop work” order issued halfway through construction.

One again, locals were outraged at the stark imposition, complaining their views were blocked and that national laws had been flouted. Spain’s 1985 Law of Historical Patrimony spells out that conservation, consolidation and rehabilitation of historical monuments “should avoid all efforts at reconstruction unless parts proven to belong to the original are used”.

“If one extreme is simple conservation, that is, leaving something to die, this is the other extreme,” Grassi conceded when he showed a visitor around the site. “In this case, out of a Roman theatre, we have created a modern and functioning theatre in the style of the ancient Romans.”

Eight years ago, after almost two decades of legal battles, the supreme court ruled the building should returned to its previously ruined state, but it remains defiantly intact – and visited by architects from around the world who shower praise on its bold originality. With Quevedo’s blunt monolith, Spain has added another landmark to its roster of stubborn anti-monuments that may prove enduring – and perhaps, with time, even endearing.