What is "chateau-sur-mer"

Chateau-sur-Mer is the first of the grand Bellevue Avenue mansions of the Gilded Age in Newport, Rhode Island. Located at 424 Bellevue Avenue, it is now owned by the Preservation Society of Newport County, and is open to the public as a museum. Chateau-sur-Mer's grand scale and lavish parties ushered in the Gilded Age of Newport, as it was the most palatial residence in Newport until the Vanderbilt houses in the 1890s. It was designated a National Historic Landmark in 2006.