The oldest building on campus is the administrative center, Whitman Memorial Building, commonly referred to as "Mem". Built in 1899, the hall, like the college, serves as a memorial to Dr. Marcus and Narcissa Prentiss Whitman. The building is the tallest on campus and was placed on the National Historical Register of Historic Places in 1974. The oldest residence halls on campus, Lyman House and Prentiss Hall, were built in 1924 and 1926. Over the next fifty years, the college built or purchased several other buildings to house students, including the former Walla Walla Valley General Hospital, which was transformed into North Hall in 1978. In addition to the seven Residence Halls, many students choose to live in one of eleven "Interest Houses," run for sophomore, juniors, and seniors committed to specific focuses such as community service, fine arts, environmental studies, multicultural awareness, or the French, Spanish, or German languages. These houses, like most of the residential architecture of Walla Walla, are in the Victorian or Craftsman style.