BIZANET, France — Here’s a game to play while driving through the Aude, that rocky, wind-swept region outside the medieval city of Carcassonne in the southwest of France: Try to spot all the ruined castles. In days of old they commanded the tops of hills and mountains, providing bird’s-eye views of the surrounding landscape and any invading marauders. But these days, centuries after falling into disuse, most are nothing more than crumbled walls that seem to sprout out of the terrain, nearly indistinguishable from the rocks beneath them.

And then there’s the Château de St.-Martin de Toques. The castle lays claim to being the only totally rebuilt privately owned Cathar castle in existence. According to the current owner, the businessman Philippe Hesnault, its origins go back further than the anti-Catholic religious movement that flourished in the region from the 1100s until the 1300s; they go all the way back to Roman times.

When Mr. Hesnault’s father, the transportation titan Pierre Hesnault, first saw the castle about 25 years ago, more than half of it lay in ruins. But something — the beauty of its setting, with 360-degree views over vineyards producing Corbières wine, the rugged power of its remaining massive walls, the romance of its drawbridge and moat, now dry — captured his imagination.

Pierre Hesnault bought the fortress, outside the village of Bizanet, and in 1990 commenced a multiyear, multimillion-dollar restoration. Or rather, re-creation. He didn’t just rebuild; he reimagined every detail as it might have been back in A.D. 978, when the mighty de Narbonne family, who gave their name to the neighboring city, inhabited the place.