Boldt Castle is a grand mansion complex on Heart Island in the Saint Lawrence River, created beginning in 1900 by hotel magnate George Boldt as an elaborate summer home and intended gift for his wife, Louise. Designed on a vast scale with multiple island buildings and a yacht house on nearby Wellesley Island, construction ended abruptly after Louise Boldt’s death in 1904, and the unfinished estate stood for decades as a romantic ruin. After the Thousand Islands Bridge Authority acquired it in 1977, extensive restoration transformed it into a major seasonal destination, preserving both restored interiors and unfinished spaces while highlighting notable structures such as the Power House, Alster Tower, and the yacht house.