Originally built as the Royal Exchange, the structure was designed by Thomas Cooley, who had won a design competition run for the project between 1768 and 1769. The exterior of the building is primarily made out of white Portland stone from a quarry in Dorset. The carved capitals were by Simon Vierpyl, and plasterwork by the stuccodore Charles Thorpe. The neo-classical building contains a central entrance hall or Rotunda, with a large dome supported by twelve columns which are surrounded by an ambulatory where the merchants strolled and discussed business meetings.