For the past 22 years, the castle at the end of Main Street in Great Barrington, Mass., has been home to an experimental school for troubled teenagers. The two-dozen students sleep in the turrets and the carriage house and play Frisbee on the lawns of the walled estate. The director’s office is housed in the drawing room with a marble fireplace flanked by five-foot statues of Hercules.

For the first time in decades, the 1888 French château, built by one of the richest women in the country, who later married her decorator, has thrown open its heavy doors to the public — the buying public, that is.

With 40 rooms, 36 fireplaces, seven turrets, and a dungeon situated on 61 acres, the Searles Castle hit the market this summer for $15 million. The sale attracted a cadre of potential buyers to the small Berkshires town known for nearby cultural institutions such as Jacobs Pillow and Tanglewood. A European engineer’s offer for more than the asking price has been accepted. Once the deal is closed, it will be the most expensive residential sale in the area’s history.

“In the last five years, a number of properties have been built that are in excess of $10 million,” the president of Wheeler and Taylor Realty, Joseph Carini, said. “The sale of Searles Castle will be the beginning of something that the Berkshires hasn’t seen before,” he said, adding that the most interesting thing about the castle is its history.

In 1878, Mary Hopkins inherited tens of millions of dollars when her husband, the Central Pacific railroad magnate Mark Hopkins, died in Yuma, Ariz. She was living in San Francisco at the time, finishing the decorations on her Nob Hill home, when she asked her decorator, Edward Searles, a mill worker who ascended the ranks to decorate the Vanderbilt homes, to design the estate she was building in Great Barrington.

While the mansion is not technically a castle — by definition it would require a moat — Hopkins spent more than $1 million, an enormous sum at the time, to build an estate fit for a king, or queen. She hired McKim, Mead, and White and hundreds of workers from around the world — importing roof shingles from Belgium and Belgians to install them, walls of marble from the Atlas Mountains in North Africa and the hills of Italy, and even using blue dolomite from her local quarry on East Mountain.

Because English oak was difficult to find even in Britain, Ms. Hopkins had two oaken ships sunken off the coast of Scotland towed to America and disassembled to secure the fine wood for her grand hall. The music room is capped with a dome reaching 42 feet and a circular window opening on to the third floor to flood that part of the house with music. (W.E.B. Du Bois worked one summer during the construction as a timekeeper for $1 a day.)

In a move that shocked upper crust society, Ms. Hopkins later married her handsome decorator, who was 22 years her junior. The couple wed at the Trinity Church in New York City in 1887. The house was finished the following year.

When Hopkins died four years later, she left everything to Searles, who quickly gutted the 6,000 square foot estate and moved to Rhode Island with his male personal assistant.

The sale of the Searles mansion — oddly named for the decorator rather than the woman who built it — attracted hundreds of queries. “The global market thought it was a bargain at $15 million,” an agent with William Ravies, Kristine Giradin, who is selling the property, said. The weak dollar has attracted numerous inquiries from Europe, including a few from “people who collect castles.” One prospective buyer wanted to build a winery (the dungeon wasn’t considered cold enough), while another wanted to convert it into condos.

The castle has changed hands only about a half dozen times since it was built. Its last buyer, Dr. Thomas Bratter, stumbled upon the place in the 1980s when his car broke down on his way to a concert at Tanglewood from Scarsdale. The only open Buick dealer was in Great Barrington. Looking for a way to kill time, he said he heard about a castle for sale and went to check it out. He soon traded $450,000 for a set of keys to the castle and opened the John Dewey Academy.

Asked if he would miss the place, Dr. Bratter looked around — a chandelier hovers above his office that looks out over a terrace reminiscent of a French chateau — and noted that it wasn’t really to his taste. Dr. Bratter, 68, said it was time to sell; he plans to move the school — which charges $80,000 a year — into another location in the Berkshires.

Early last century, a number of local businessmen got together to buy the place for about $65,000, and once inside they found a chest or two filled with linen and silver worth just as much. Dr. Bratter says there are three safes that have yet to be opened.

The dungeon is off limits to the students, although according to local historian Lila Parrish, who published an exhaustive history of the estate, “looking like dungeons, the subbasements were used for more humane purposes.”

The students say they are going to miss living in the castle and that they respect that the place was all hand made. They won’t miss everything: One student, sitting in the front room floored with moriah marble and covered in ornate oak paneling, said, “It takes a lot of time to do all the dusting. We have to get in the curves with Q-tips.”