You can find some rules for your game :
Caesar cipher (video) : code breaking, frequency analysis,...
The ENIGMA machine explained : Word Science Festival. Simon Singh (video YouTube).
The enigma machine :
Article : Morse et Wheastone (site : visite Arts et métiers.free).
Cryptography in our lives : TED video from Tal Rabin, Manager and research staff member of the Cryptography Research Group at IBM's T.J. Watson
Cryptography for everyone : John Voight at TEDxUVM (he is assistant professor of mathematics and computer science).