Here is a complete list of all the known secret messages hidden throughout Duke Nukem 3D. I included them because I find secret messages to be very interesting. Try including them in your own levels! Two of my tutorial maps have them...
Episode 1
Level 1
Level 3
Level 4
Level 5
Episode 2
Level 1
Level 2
Level 3
Level 7
Level 8
Episode 3
Level 1
Level 3
* "...not sure of the spelling, but that is a tribute to the movie Excalibur. I believe it's Gaelic for dragon's breath and was a tie-in to the steam being emitted at the message." - Richard Bailey Gray (the Levelord)
Level 4
Level 5
Level 6
Level 7
Level 8
Episode 4
Level 1
Level 3