The Restore Rochester Castle Campaign wants to save the walls, floor and roof of the 12th Century building.
A section of the outer wall of the castle had to be rebuilt last month after collapsing because of heavy rainfall.
Campaigner Jon O’Donnell said the medieval castle was “decaying faster and faster”, and access was restricted because of falling stone.
He said: “Access around the base of the castle is now no longer possible due to falling ragstone.
“People are now only able to enter with the help of safety netting over the entrance.”
Mr O’Donnell, who chairs the Restore Rochester Castle Committee, added: “We believe we represent the Big Society – people stepping forward to take ownership of our castle and saving it for future generations.”
Campaigners plan to raise funds for the castle’s restoration, raise awareness about its plight and petition the Department of Culture, Media and Sport for support.
Source: BBC News