The Castle, also known as Usen Castle, is a historic building at 415 South Street in Waltham, Massachusetts. The Medieval structure, built in the style of a Norman castle (its apparent inspiration was Cavendish Castle in Ireland), was built in 1928 by Dr. John Hall Smith, founder of the Middlesex College of Medicine and Surgery. After that school closed in 1945, its campus was acquired by the founders of Brandeis University. Brandeis has used the building for administrative offices and student housing.
The building was listed on the National Register of Historic Places in 1979.