how do you climb down a ladder in inquisition?

You can also just jump down, you don't need to climb down. You cannot die from jumping, you will just lose a lot of hit points (depending on the height of the jump, not much for a small drop) outside of your home base. In your home base, you can jump from any height without consequence.

Note if you land among a bunch of enemies  and start combat and lose enough hit points, then you may enter the knocked out state.