Use the following trick to get siege equipment on walls (not the mounted ones that are bought, but the ones that you make). Build a Gatehouse connected to a wall and build stairs far away from this. Send the equipment that you want through the gate from the outside to the inside. As soon as they are inside the gate, shut it and they will appear on the wall heading for the stairs. Stop them where desired (but not on towers as they will disappear), then you have a nice weapon on a wall.