Dumas did not write his novels alone – he had a help, including that of a man named Auguste Maquet. (Source)
The historical figure on whom the Man in the Iron Mask was based actually wore a black velvet mask. (Source)
In the late 19th century, Monet frequently painted at Belle-Isle.
It is said that Dumas cried while writing the death scene of Porthos. (Source:Zipes, Jack. Afterward. The Man in the Iron Mask. By Alexandre Dumas. Signet Classics, 1992)