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.

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.