Harthill Castle is a pink-washed Z-plan twin-tower building, seven storeys () high, made of solid granite, and set in woodland and farmland in Oyne at the foot of Bennachie. The ground floor is vaulted and has arrow slits and musket holes for windows. The next two storeys are partially vaulted. There are circular staircases in each tower, each rising counter-clockwise. The Great Hall has a fireplace that predates the building and was probably brought to the property by the Leiths from their previous home.