The former Sheffield Castle was located in the center of Sheffield at the confluence of the Rivers Don and Sheaf. The castle was a significant building and centre of power in Sheffield before its demolition during the English Civil War. Despite its size and importance in Sheffield, and in its role as a prison for Mary Queen of Scots for many years, no contemporary plans or illustrations exist of the castle. This talk will summarise our knowledge of the layout and form of the castle based on excavations that have been undertaken over the last century. It will summarise and contrast the methods and results of the work undertaken in the late 1920s by Armstrong, in the 1950s by Butcher and the 1990s and 2000s by ARCUS. The talk will outline what we know of the layout of the castle based on the excavation results, and documentary sources, and discuss the research potential of the site in light of current proposals to redevelop the area.