PLIEUX, France — Though the writer had already lived in his castle for a quarter of a century, it was only three years ago that he finally restored it to its original purpose as a fortress.

The writer, Renaud Camus, rebuilt the top 10 feet of the 14th-century tower, giving him an even more commanding view of his surroundings: the village of 40 souls below; the Pyrenees, faintly visible some 100 miles south despite the midsummer haze; and, in every direction, the peaceful, rolling hills of the “eternal France” that he describes as under assault from what he calls hordes of immigrants.

Up in his castle, the France that Mr. Camus imagines has made him one of the most influential thinkers on the far right in his own country and elsewhere. In his writings, he describes an ongoing “invasion” of France by immigrants bent on “conquest” of its white, European population. To him, the immigrants are “colonizing” France by giving birth to more children and making its cities, towns — and even villages — unlivable.

Others have espoused similar ideas. But Mr. Camus’s portrayal of demographic change — le “grand remplacement,” or the supposed “great replacement” of France’s original population by newer arrivals, mostly from Africa — has become an extremist talking point, cited by mass killers in distant parts of the world.