Gymnasium

The building is a prison gymnasium constructed around 1865, likely designed by Captain W. Crossman, Royal Engineers, who was the general designer for The Citadel. It is built from Portland stone, primarily rusticated, and features an asbestos-cement slate roof. The gymnasium is a single-storey rectangular structure.

The east gable showcases three arched openings above the entrance, which contain paired sash windows, and has a low stack at the gable apex. The south front, partly obscured by a late 20th-century flat-roofed addition, has a layout of 2:2:3 windows with arched heads over paired casements and lunettes, all set on heavy stooled cills. There is a central eaves stack with a tall shaft and block modillion capping. The west gable is constructed in heavy rock-faced masonry and features a central buttress, along with a plinth and raised coped verges. The interior has not been inspected. This building is characterized by its vigorous detailing, typical of the Royal Engineers' work at The Verne.