Castel Valer, a historic fortress nestled in the foothills of the Dolomites mountain range in northern Italy, is going under the hammer Sept. 8. This is the first time the property will change hands in nearly 650 years.

Built over half a millennium, from 1200 to 1700, the castle combines architecture from the Romanesque, Gothic and Baroque eras, according to Concierge Auctions, the luxury real estate auctioning house based in New York.

Although it has undergone restorations to incorporate all modern facilities over the last 20 years, the castle still retains a cloister of pillars dating to the 16th century Council of Trent, in addition to an octagonal tower used as a World War I artillery base for German and Italian military.

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Count Spaur zu Flavon und Valer, whose family has owned the castle since the 14th century, did not disclose reasons for the sale.

“The home holds a lot of history for both my family and my country. I especially love the details in the Gothic chapel and have a fondness for the Ulrich hall, which was named after my ancestor,” he said in a statement. “I hope the new owners of the home will enjoy the property as much as I have and further add to its fruitful timeline.”

The castle sits on 18.5 acres of terraced gardens, orchards and forest overlooking the valley below. The 35,520 square feet of living space comprise more than 80 rooms, including 15 bedrooms, 14 bathrooms, five kitchens, a banquet hall, a library and two chapels.

The Late Gothic-style Saint Valerio chapel, embellished by frescos from 15th century Italian masters Giovanni and Battista Baschenis, has been designated Europe's Best Renovated Chapel by Christie's and Europa Nostra.

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Being sold fully furnished, the castle is fitted with bespoke antique furniture, glass chandeliers and Venetian mirrors. The auction is being held without reserve, which means there is no minimum bidding price and the highest bidder will become the new owner.

Concierge Auctions has begun previews of the property. Interested bidders typically are required to wire a deposit between $100,000 and $500,000 in addition to a reference from banks or other financial institutions to prove their wherewithal. The money is refunded if they don’t have the winning bid.

The property was previously valued at €35 million (or $38.8 million), according to Concierge Auctions.

The castle’s location affords immediate access to beautiful lakes, quaint villages and a world-famous ski resort. Also within easy reach are the historic Italian cities of Verona, Venice and Milan, as well as the international centers of Zurich, Munich and Salzburg.

Write to Fang Block at fang.block@dowjones.com