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]