/\ Yeah what he said. And make sure you leave some ladder sticking up at the top, if you bring it down flush with the platform you wont be able to run up to it and go strate down the ladder, if its frush you wont stick to the ladder and just fall down.
Remember you can over lap ladders to make them bigger if you need to.
Raise the ladder so that the green circle and blue arrow are at the location you want the top of the ladder to be. This will allow you to zoom in and place the ladder very precisely. Rotate the ladder so that the blue arrow points out. When you have the ladder in position, lower the ladder so that the top of the ladder is slightly higher than the surface. In the sniper tower, please place the ladder so there is a place to get off the ladder.
MOST IMPORTANT: TEST YOUR MAP! Test ladders, docks, hill pitches and everything else before you publish the map.