In 1661 Sissinghurst's owner, Sir John Baker died, leaving four daughters as heiresses. By 1730 there were no descendants left of the Baker family and Sissinghurst was sold to Sir Horace Mann.
Sir Horace Mann never used Sissinghurst as a home and spent the whole of his life from 1737 onwards in Florence. Rather than sell up, he leased Sissinghurst to the Government for use as a prison, when up to 3,000 French prisoners were incarcerated during the Seven Years' War (1756-63). The prisoners wrote home to their families, often referring to Sissinghurst as Chateau de Sissinghurst; the name stuck and its thanks to these imprisoned seamen that Sissinghurst is now called Sissinghurst Castle.
French naval officers were housed in the tower. To while away their time they drew graffiti depicting their ships in full sail on the soft walls of the tower. These can still be seen, along with names and dates, etched over the years that prisoners were held here. Lower-ranked ratings were forced to live in much more squalid conditions, often having their clothing and food rations cut for petty crimes by the English military guard who had a duty of care to them.
This image was drawn by one of the prisoners, we can tell this as the guards are ones seen to be causing harm to inmates.