specifically, the HORDE part, as I have found several walk through sites, and even a couple of hacks n cheats, which I do NOT use
however, I have found what can only be called an Easter Egg

to start, you are going to be in the HORDE mode, the basic death match, you cannot win, only kill as many Zombies as possible, however, in one map - Waves of Despair - I have discovered a path to access outside the building that contains the basic battleground.
after setting up, make your way to the far end of the building, at the end of a tunnel, then climb the stairs, here you will find access to dynamite and trip mines, as well as the PREACHER weapon, which is the big gun in this game
to find the 'escape' after climbing the stairs, you enter the room, turn right, and climb the next stairs, turn 180 degrees, and then RUN down the stairs and up the second set, then as you hit the top of the second set, and you are next to the wall on the L/H side, you will see a CLIMB option flash on screen, hit the E button to action it, and you should then climb out of the kill zone
this will be a small rooftop area, you are safe here for the first few levels, as they Zombies need to catch you, the ones with guns appear later in the game, you will run out of ammo, and the best place is to jump off roof towards a truck on the outside of the building, run to the truck, then carry on in that direction, and run to the end of the building, turn left, and continue to end, at the next corner you will find rifle ammo, and if you run past that and past the stairs, you will find ammo for the secondary weapon too, climbing the stairs here, and you will find a gun emplacement, the walls are not a barrier to us, but will stop the Zombies, so again, you can sit in comfort, and kill as many as you can in safety, until the ammo runs out, and you need to reload via the ammo dumps below.
you may need to access the building again too, to reload with mines etc, just remember the way back to the safe zones is ONLY available as set out at the start
have fun, I will add details of this level specifically, as I have had some fun here, and setting mines and trip mines to create super traps is just a great laugh