Similarly to Doom and Doom II, Doom 64 will display built in demos of gameplay upon idling at the main menu for a while. There are three of these demos already unlocked and playable, and a fourth one that requires a secret password.
Demos[]
The normal demos are of the levels: MAP03: Main Engineering (Doom 64), MAP09: Even Simpler (Doom 64), and MAP17: Watch Your Step (Doom 64). The secret demo is MAP32: Hectic (Doom 64), and the code that you need to enter in order for the demo to play is 3CT1 CD3M 0???.
Trivia[]
Note that Hectic's demo only plays after all of the regular ones have.
In all of the demos except for Hectic, the marine dies.