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What follows is a picture that I whipped up of what my first ladder is going to look like. I have reviewed the suitable threads in a number of messagebases and even followed along with the video tutorial, which I thought was very helpful at explaining the concept.
Lep's right about the func_useableladder thingy. You make that, set the bottom and the top (by dragging the circle on the entity, it doesn't matter which way round) and then name the ladder. The info_ladder_dismounts are in good places, but you don't set the Parent to the name of the ladder, there's another thing with a dropdown box, something like Ladder Name, I can't remember.
And yeah, the upper ladder point needs to be raised up. It's best if the bottom of the top ladder point is a tiny bit higher than the top ledge (if that makes any sense).
Hope that's helpful, it's not as complicated as it seems.
Edit: If I've confused you, just look in the SDK reference docs, the ladder tutorial thing there is very helpful.
