Okay, here are at least the demo cheats:

 Press [Esc] while playing to enable the codes. The magic words are:

 CHOJIN           - immortal mode.
 AMMO             - restore ammo for all weapons.
 WEAPON           - gives all weapons to player.
 INVISIBLE        - gives 2 minutes of invisibility.
 FULLMAP          - allows you to see full map of the current level.

 KILL             - kill all monsters on the level.
 REANIMATE        - (debug) reanimate all monsters.
 FREEZE           - (debug) freeze all monsters.
 RESPAWN <#time>  - set the monsters respawn time.
                    (#time=0 means no respawn)

 SLOW             - (debug) slow mode.
 REVERSE          - swap right and left sound channels.
 DEPTH            - distant fade.
 SHADOWS <number> - set the upper limit of shadows casting by 3D object.
 GO <#level>      - warp to specified level.

 * Command without a parameter returns its current state.