Sycamore's historic district was approved and entered in the National Register of Historic Places in 1978. The classical revival style Dekalb County Courthouse serves as a keystone for the district's other 226 structures. Built in 1903, the courthouse is distinguished for its marble and stained glass interiors. It was preceded by two earlier buildings, the first erected in 1839. The unique Civil War monument on the lawn predates the present structure. Diagonally east opposite the courthouse stands the Byzantine style Sycamore Public Library, built on land given in 1903 by F.B. Townsend, an early Sycamore philanthropist. Between these two historic buildings runs Main Street, once a Potawatomi Indian trail and now a major thoroughfare within the district. Sycamore is one of a group of small Illinois county seats that survived into the 20th century preserving both the fabric and the sense of their earlier history. The courthouse, the central commercial district, and fine residential areas, especially on Main and Somonauk Streets, are significant for their special architectural merit.