Looming over Heart Island, across from Alexandria Bay, is the gloomy Boldt Castle (315/482-2501, www.boldtcastle.com, 10 a.m.–6:30 p.m. daily May–Oct., with extended hours July–Aug., adults $7, children 6–12 $4.50, plus the cost of the boat ride over), 127-room replica of a Rhineland castle. The castle was built by George Boldt, who came to the United States from Prussia in the 1860s. The son of poor parents, Boldt had tremendous industry and skill, and eventually became the most successful hotel magnate in the country. Both the Waldorf-Astoria in New York City and the Bellevue-Stratford in Philadelphia were his.

Boldt was deeply in love with his wife, Louise, and built the castle around the turn of the 20th century as a symbol of his love for her. The castle was to be their summer home, and he employed the finest craftspeople, instructing them to embellish the building with hearts wherever they could. Boldt even had the island reshaped into the form of a heart.

Then in 1904, when the castle was 80 percent complete, Louise passed away. Boldt sent a telegram to the construction crew to stop work immediately, and never set foot on the island again. The castle was abandoned and allowed to deteriorate.

Finally, in 1977, the Thousand Islands Bridge Authority bought Boldt Castle, partially rehabilitated it, and introduced it to the tourist trade. Today, hundreds of visitors traipse through it daily, but all the activity in the world can’t erase the castle’s haunted and wildly romantic feel. All around are broken moldings, half-finished walls, and cold, echoing hallways. In the former ballroom, exhibits explain the castle’s history.

A shuttle boat operates from the castle to the Boldt Yacht House (adults $4, children 6–12 $2), perched on a separate island nearby. Completed before Louise’s death, the boathouse contains three original spit-and-polish boats and restored living quarters furnished with handsome antiques.