Boldt Castle is a major landmark and tourist attraction in the Thousand Islands region of the U.S. state of New York. Open to guests seasonally between mid-May and mid-October, it is located on Heart Island in the Saint Lawrence River. Heart Island is part of the Town of Alexandria, in Jefferson County. Originally a private mansion built by American millionaire George Boldt, it is now maintained by the Thousand Islands Bridge Authority as a tourist attraction.
George Boldt, general manager of the Waldorf-Astoria Hotel in New York City and manager of the Bellevue-Stratford Hotel in Philadelphia, and his family enjoyed an earlier frame cottage on Hart Island (the island's original name) for several summers, which they greatly expanded. In 1900, George Boldt launched an ambitious construction campaign to build a huge masonry structure, one of the largest private homes in the United States. He engaged the architectural firm G. W. & W. D. Hewitt and hundreds of workers for a six-story "castle" as a present to his wife. In addition, four other masonry structures on the island are architecturally notable. Equally distinctive is a huge yacht house on neighboring Wellesley Island, where the Boldts had another summer home and a vast estate, incorporating farms, canals, a golf course, tennis courts, stables, and a polo field.
The construction of Boldt Castle ceased abruptly in early 1904 after the death of Boldt's wife, Louise Kehrer Boldt. Boldt never returned to Heart Island, but continued to spend summers in the 1000 Islands until his 1916 death. For 73 years, the castle and other stone structures were left exposed to the harsh winter weather and occasional vandals. The Thousand Islands Bridge Authority acquired Heart Island and the nearby yacht house in 1977, for one dollar, under the agreement that all revenues obtained from the castle operation would be applied towards restoration, so that the island would be preserved for the enjoyment of future generations. In the five decades after acquiring the property, the Thousand Islands Bridge Authority spent around $15 million for restoration and improvements with work continuing annually. The initial goal of the restoration of Heart Island was not to finish what had not been completed, but to restore the island to the state it was in when construction was halted. Improvements have surpassed that goal, as a stained glass dome, marble floor, grand staircase woodwork, butler's pantry, maid's and servant's dining rooms and kitchen were restored. As of 2020, over $50 million has been spent on restoration and rehabilitation work on the Castle and surrounding structures.