Gillette began work on Seventh Sister with an eye toward creating a retirement estate for himself. Between 1914 to 1919, he took an active role in designing it and supervising its construction. The castle is made of fieldstone and boasts 24 rooms, set on over 100 acres along the shore of the Connecticut River. Its asymmetrical layout and imposing presence evoke the picturesque Gothic ruins of Europe.
Inside, the castle features a more Arts and Crafts aesthetic with customized woodwork, furniture, and lighting fixtures--Gillette had an unwavering attention to detail that led him to create numerous personalized touches. The castle has 47 doors in total, and no two are exactly alike. There's a small secret room with a fireplace. A series of mirrors allowed Gillette to spy on guests. He had a miniature train line installed on the estate. He also had an impressive collection of cat toys, meant to keep his 17 cats happy. According to architect David H. Barkin, There's nothing typical about this building. Everything about it is unique (quoted in the NYT).