The design of the entry steps that led up the slopes of the Acropolis hill and then merged with the steps and central ramp of the Propylaea continues to be a source of debate among scholars (if not in their writings, then certainly in their depictions of the Acropolis). It is not our intention to wade into these discussions too heavily, other than to seek a resolution that jives with the (architectural, archaeological, epigraphic, and ancient textual) evidence as reliably as possible.
There seems little archaeological evidence one way or another for how the Classical entrance steps would have been configured, and there are no ancient writers who talk about the steps in any architectural detail. Several schools of though have emerged, among scholars, the Greek archaeological service representatives, and architects, who have all put forth their interpretations (listed here in no particular order):
a) a long series of steps with a central narrow ramp meeting the Propylaea central corridor and steps;
b) a long series of very deeply cut steps with no ramp; and
c) a set of several steps with narrow landings between them off of which run various side steps and ramps.
It is one of those details of the Acropolis that is often taken for granted, but when it comes time to create an accurate 3D model, we need to ask specific questions of the data and focus on nuances of use that are often glossed over in hand renderings or general discussions. Things like: whether there was a central ramp, whether there was a full-width ramp-stair, whether there were landings, whether there were side paths leading into and away from the central steps, how steep the angle of steps were, how wide they were. These are not trivial matters when one considers how the actual ancient processions, casual worshippers, workers, and officials with animals, goods, and building materials would have ascended to the top.