Mostly the units walk a specific amount of time on the wall.. you could set the distance of the wall-walk to some created areas.
Place your wall and create a region at the beginning of the wall and at the end of the wall.
You could for example trigger the next effects with it, when a unit enters that region, replace it with the same unit, but with "flying" as movement type and play the wall-walk animation (you need to add, or you must rotate the unit, but I don't know how). Then just simply set the height of the unit higher and higher till you have a good height to walk on the wall.. then just simply let it move to the end of the wall and do lower the "flying" height of the unit.
Should do the job..
You could also set a complete region in front of the wall and if the unit casts an ability (wall-walk) while it's in that region, let it walk your wanted distance on the wall like mentioned above.