A mobile weapons platform, is exactly that. mobile.
Not on a wall
If I wanted to but a vehicle on a wall I would just build a GP instead. Maybe it would be an idea, to be able to wall upto the GP.
but at the same time destroying the GP would knock out the wall segments around it.
Another thing. OP2 is a 2D, top down game, how do you know that the cliffs arn't hundreds of meters high, i mean a wall in OP2 isnt going to be built 100's of meters high is it, so how the hell would it be feasible to create a bridge between cliffs using it?