When the French gallerist Yves Gastou bought the Ermitage de Douce-amie — or refuge of a sweet friend — a follylike crenelated castle on the outskirts of Biarritz in southwest France, in 1990, it had the air of a place frozen in time. Hidden within a forest of bamboo and pine, bay and oak trees, the 5,380-square-foot house was built in 1900 as a retirement home for a member of the imperial court of Napoleon III, or so Gastou believed. The house had changed hands only a few times since: The woman from whom he purchased it was among the last ladies in Biarritz to travel by horse and cart.

For Gastou, a decorative art and antiques dealer who died in 2020 at age 72, the house, with its faded pale blue cement facade and roughly 40-foot-tall turret, fulfilled romantic fantasies of knights and fortified towers that had taken root during childhood trips to Carcassonne; he grew up near that medieval walled city in Limoux. As an adult, he lived for most of the year in an apartment on the Quai Malaquais on Paris’s left bank that was a short walk from the namesake gallery he ran from 1986 until his death. The hermitage was his summer hideaway, a place for monthslong vacations with family and friends. And if his Paris home was an expression of his appetite for modernity — it epitomized his eclectic tastes, with Cubist midcentury furniture by the French sculptor Philippe Hiquily and acrylic 1980s-era pieces by the Japanese designer Shiro Kuramata — the castle offered a retreat into the past.

Over the decades, Gastou gradually restored the building, referring often to the architect’s plans and a 1930s painting of the home’s exterior that he discovered on the property. Working without a decorator, he hoped to keep the house as true to its origins as possible, while also using it as a repository for his many wide-ranging collections. Besides transforming the adjacent stables into a four-bedroom guesthouse and installing a lap pool on the sloping two-acre grounds, his main concession to modernity was upgrading the kitchen, which sits at the rear of the house on the first floor, beside the dining room — and adding a bathroom on each of the four floors.

Most striking among these is the basement bathroom, whose floors and walls are entirely clad in white-and-green veined Vert d’Estours marble and white-flecked deep-gray Saint Anne des Pyrenees marble in an ode to Villa Kerylos, the famous turn-of-the-20th-century ancient Greek revival-style mansion on the French Riviera. Guests returning from the beach, a 25-minute walk from Gastou’s home, and wanting to freshen up would be greeted by a large bronze statue of Apollo, perched on a pedestal in a corner and modeled after a Roman version at the British Museum. It was juxtaposed with a blocky marble and terrazzo chair by the 1980s-era Memphis Group designer Ettore Sottsass, who oversaw the design of Gastou’s gallery and whose radical, irreverent work the dealer affectionately likened to a punch in the face.

Gastou liked to express his love of the past, in part, by enlivening it with the shock of the new. A 1960s life-size mirrored figure by the Italian artist Claudio Platania and the French fashion designer Pierre Cardin stood on one of the home’s landings, reflecting the angular carvings of the original pine banisters. In the garden, a 1990s galvanized steel winged bench by the British designer Tom Dixon contrasted starkly with the classical silhouettes of the pair of stone urns that flanked the pool.

“The house is a cabinet of curiosities,” says Gastou’s son, Victor Gastou, who now runs his father’s gallery and owns the castle with his younger sister, Mathilde Dahdi-Gastou. As a teenager, rather than lounging by the pool, he would sit for hours at his father’s large leather-topped walnut desk in the library on the first floor and peruse the room’s myriad treasures. Situated just off the main entranceway, past a mound of pale gray stones that Gastou would sometimes add to after a trip to the beach, the library suggested the study of a grand tour-era gentleman. Antique photographs of rural scenes and regional portraiture, picked up at local flea markets, hung on the pale yellow walls. A mid-19th-century gilt and bronze clock shaped like a smiling devil sat on the desk, beside which loomed vast midcentury oak and iron shelves by the French Art Deco designers André Arbus and Gilbert Poillerat. And lining the shelves, along with marble urns and antique leather-bound books, were items from Gastou’s collection of religious relics: crucifixes, ex-votos, Virgin Mary crowns and mother-of-pearl-mounted flaming hearts.

Gastou’s bedroom, at the top of the castle’s turret, was similarly decorated with the rosaries, sacred hearts and holy water fonts he gathered throughout his life. Though he attended Mass as a child, he was spiritual rather than religious and was drawn to the talismanic qualities of these objects. He would collect used crucifixes from the nearby Convent of the Bernardines each time the sisters replaced their crosses, worn from years of prayer. “My father rescued objects in the same way some people rescue animals,” Victor says. “Each one told a different story. He was touched by the magic of their patina.”

Now almost entirely emptied of its contents — which, together with the art, objects and furnishings from Gastou’s Quai Malaquais apartment, will go on sale at Sotheby’s in Paris on March 19 — the house is ready to begin yet another chapter. While Victor sees the auction as a chance to share his father’s story, giving his objects new life, he also wants to bring the castle into the 21st century. He plans to renovate the house, then spend vacations there with his own son, Cesar, now 7 months old. And though Victor’s own tastes are more minimalist than Yves’s, as he begins to consider making changes to the home, one phrase of his father’s, his motto for collecting, continues to echo in his head: “When you open your eyes, you can see beauty.”