The bodies of the two men were found naked in the partially dry moat surrounding the Vauban Fort in northern France on Thursday morning.
Their belongings were found on the battlements 40ft above, leading police to believe they were having se*x when they fell.
One 31-year-old man was found in the dry part of the moat, while his partner was discovered shortly after in water only 5ft deep.
Both men are believed to be from Normandy and were on a holiday visiting the castle when the accident occurred.
France’s Le Figaro paper quoted a police source as saying the men were probably ‘making love – and something went wrong’.