Complete the game under the indicated difficulty setting to unlock the following options at the "Extras" menu, usable in single player mode.

Option to unlock	How to unlock
Concept Arts	Normal difficulty or higher.
Development Timeline	Normal difficulty or higher.
Duke 3D Freeze Ray (freeze ray fires bouncing ice projectiles)	Normal difficulty or higher.
Duke Sound Board	Normal difficulty or higher.
Game Speed (adjust game speed)	Normal difficulty or higher.
Grayscale Mode (black and white graphics)	Normal difficulty or higher.
Head Scale (adjust head size of enemies and NPCs)	Easy difficulty or higher.
Infinite Ammo (cheat; self-explanatory)	Normal difficulty or higher.
Instagib (cheat; enemies killed with one shot and always gib)	Normal difficulty or higher.
Invincibility (cheat; no damage from bullets or explosions)	Normal difficulty or higher.
Mirror Mode (graphics are mirrored horizontally)	Normal difficulty or higher.
Movies	Normal difficulty or higher.
Screenshots	Normal difficulty or higher.
Triptych Office Photos	Normal difficulty or higher.

Note: Enabling the CHEAT options will prevent Ego points and Achievements from being earned.