Aramis' statement, "I am a genius, not an engineer," is a pun in the original French ("Je suis un génie, pas un ingénieur").
MGM discovered the audience the movie attracted was "directly related to appeal of Leonardo DiCaprio", being 55% female and 46% under the age of 25.
This is the third in a trilogy of the Alexandre Dumas père novels about the adventures of Athos, Porthos, and Aramis, which also include 'The 3 Musketeers' and 'Twenty Years Later'.
Alexandre Dumas père, the author of the book for which this is based, adapted his novel on a historical account of a mysterious prisoner in the Bastille who was forced to wear a leather mask for the two years between his imprisonment and subsequent death.