Enable Console
     Select  Video/Audio  Menu  -->  Advanced  Options  -->  Checkmark
Console.  Once playing the game, bring down the console by pressing ~.
Enter any of the following codes at the console:

Code                      Result 
god                       God Mode 
wuss                      Get All Weapons 
notarget                  No Target Mode (Enemies Don't Attack) 
noclip                    No Clipping Mode (Walk Through Walls) 
give all                  All Weapons, Items (Health, Water, etc) 
health                    100 Full Health 
hide                      Invisibility 
show                      Invisibility Off 
killclass [classname]     Kill All Enemies of [classname] 
health 100                Full Health 
eventlist                 Show All Possible Commands 
map #                     Switch to Map # (See List Below) 

Map Listing

blood
cemetary
cliff1
cliff2
creeperpens
end
fakkhouse
fog
gruff
gruff_cinema
homes1
homes2evil
homes2good
intro
landersroost
oracle
oracleway
otto
over
shield
swamp1
swamp2
swamp3
towncenter_evil
towncenter_good
training
under
water
zoo