Construction on Boldt Castle began in 1900 at the request of millionaire hotel entrepreneur George C. Boldt as a memorial to his beloved wife Louise. Boldt Castle was created to be their summer fantasy house. A castle worthy of Europe, erected in the middle of the 1000 Islands. Mrs. Boldt, on the other hand, died abruptly just months before the castle's completion. Mr. Boldt was distraught and promptly halted all work, leaving the site unoccupied for more than seventy years.

On Hill Island, which is essentially on the border between the United States and Canada, there is a concrete observation tower called the 1000 Islands Tower. However, it is officially in Ontario, not New York State. The tower, which was built in 1965, affords panoramic views of both nations' Thousand Islands.

Zavikon Island is home to the world's shortest international bridge (32 feet). It joins a Canadian and an American island in the midst of the Saint Lawrence River. The boundary agreement negotiated between Canada and the United States states that no island can be divided into two territories. Two-thirds of the islands are Canadian, however the overall area of the Canadian islands is roughly equal to the American territory.