Southern Illinois University is a public research university in Carbondale, Illinois. Chartered in 1869, SIU is the oldest and flagship campus of the Southern Illinois University system. SIU enrolls students from all 50 states and more than 100 countries. Originally founded as a normal college, the university today provides programs in a variety of disciplines, combining a strong liberal arts tradition with a focus on research. SIU was granted limited university status in 1943 and began offering graduate degrees in 1950. A separate campus was established in Edwardsville, Illinois in 1957, eventually becoming Southern Illinois University Edwardsville.

"Altgeld's castles" are buildings in the Gothic Revival style at five Illinois public universities, all built at the initiative or inspiration of Illinois Governor John Peter Altgeld During his term as governor (1893-1897), Altgeld expressed certain opinions on how buildings should be erected in the state of Illinois. In his second biennial message to the state legislature, he discussed how buildings were being constructed without consideration for their outward appearance. He stated that it was time for buildings to become more aesthetically pleasing in addition to being functional, and he suggested the "Tudor-Gothic style" as the most inexpensive way to do this. Consequently, several of the state universities in Illinois erected buildings which resembled castles in his honor. College folklore states that these buildings have aligned corridors and rooms so that they could all be "put together" to create one large building, but there is no evidence to support this rumor.