The case of a wall between courtyards, and that wall changes - whether because the ground changes or the wall changes. For example, if you have a basin right next to the wall - which the Gemara develops and refines, when the original case doesn't hold up to scrutiny - from adding dirt, a rim, a trowel, and a comparison to turnips. Alternatively, bringing a ladder to the wall - whether an Egyptian ladder or one from Tzur.... The ladder here seems more permanent than we think of the usual ladder. So the ladder becomes a passageway, depending on its size, and diminishes the height of the wall.