Opened to the public only since 2003, this dramatic medieval castle is worth going out of your way to see. It's a stunning creation, made of the same granite used for Boldt Castle and erected at the same time in the first years of the 20th century. Unlike its sister castle, however, Singer was actually occupied, and it's still furnished with period pieces. Its builder and owner, Frederick Gilbert Bourne, was a self-made millionaire who became the director and president of the Singer Sewing Company at age 36. He bought the island and built this 28-room gem with a four-story tower as a retreat for himself and his family. It's a sprawling and beautiful, with plenty of parking space for his boats and oddities like underground passageways and dungeons. You have to take a tour, but it's an interesting one: Guides point out Bourne's secret passageways and peepholes used to spy on guests.