Hidden door -
Enable cheat mode, then activate the "/map redE3M5" code to advance to episode 
3, mission 5. Locate the door that does not have a handle in that level. Enter 
the "/noclip" code and walk through the door to find another door covered with 
smiley faces.

Cheat Codes -
Load the game with the "hl.exe -dev -toconsole" command line. Then while playing 
a game, press ~ to display the console and type "sv_cheats 1" to enable cheat 
mode. Then, press ~ and enter one of the following codes to activate the 
corresponding cheat function:

Result | Cheat Code
God mode | /god 
All weapons with full ammo | /impulse 101 
No clipping mode | /noclip 
Opponents ignore character | /notarget 
Third person view | /thirdperson 
Adjust gravity | sv_gravity [-999 - 999999] 
Obtain item | /give [item name] 
Map select | /map [map name]