It's both hilarious and pathetic to me when people refuse to awknoledge that something can be real, simply because they personally have no knowledge of it's existance. going so far as to say stuff like, "If you believe that, I've already labeled you as a child."
For those of you who didn't know, you now do...
The ladder is not a glitch, it is part of the level, Kino Der Toten (Theatre of the Dead).
After placing all of the film reels on the projector, and surviving 30+ rounds, the ladder can be purchased. It is located at the Stage, near the claymores.
This ladder grants you safety from a susspended platform, but crawlers can, and will still get to you.
It's an easter egg given to us hardcores by Treyarch. A little reward of sorts.
You didn't really think all of the little things and tricks around Zombies maps did sweet fuck all, did you?