Sully County Courthouse  The Sully County Courthouse, at Main and Ash Sts. in Onida, South Dakota, was built in 1912. It was listed on the National Register of Historic Places in 2001. It is a three-story Classical Revival-style stone building with a cupola that serves as a clock tower. The building has an entablature with a dentil molding. It was designed by architect W.M. Rich.