Quote Originally Posted by Zatoichi
If you click on a mid point on the top of the wall with a unit on the ground that's near a captured wall tower, the unit will run up the tower and assemble where you clicked.

Obviously, you have to be inside the city to do this...
Not really, you do not have to be inside the city to make a unit climb the wall. Once your tower has made the "hookup" you can click on the wall also without being inside. Units from outside will climb the tower and set up on the wall.