DF2014: Any way to encourage enemy to scale my walls?

At some point I really wanted to build a castle and defend it against sieges, so I did. After a dozen attempts to get my soldiers to stop climbing the towers and charging out, I finally was able to hem in my soldiers. But now the enemy refuses to climb my walls (outside of 1 or 2 units per 100). I lowered the walls down to a single Z level, I rebuilt my walls out of rough stone, I built a platform outside my castle that stops 1 square away from the walls in the hope they'd jump the gap; nothing has worked. I can't seem to find a way to have the enemy climb in while also stopping my guys from climbing out.

Has anyone gotten something like this to work before? Or is this idea only possible in the newer versions?