This is one good classic game it was the second game in the series the first being Wolfenstien 3D (that was also a good game!) here are some cheats

Funny Message:
When playing type JOSHUA to receive a very nice message

Jukebox:
On the opening screen press M to get a jukebox

All Everything!:
When in the game press M + L + I together and you will receive full heath all guns and full ammo but your score will drop to 0