The Ruperra Castle Preservation Trust has said the historic building should be opened to the public after it is sold.
Last week, the Grade II* listed building in Lower Machen was put on sale for £1.5million by owner Ashraf Barakat.
Mr Barakat bought the ruined castle, dating from 1626, in 1998 with plans to turn it into flats but was refused planning permission by Caerphilly County Borough Council.
A subsequent appeal was also turned down on the grounds the plans would harm the historic nature of the castle.
Welsh heritage body Cadw has ruled out using public funds to bid for the castle meaning it is likely to stay in private hands – if indeed it is eventually sold.
The Ruperra Castle Preservation Trust has said it hopes any new buyer will make the castle safe before opening it up to the public.