View looking south at the staircase on the northern slope of the Herodium where the "open" portion of the staircase transitions to the "enclosed" portion that leads into the interior of the top of the Herodium.
It looks like there is a drainage canal, in the center of the image, that led out from the summit. It was added at a time after the staircase went out of use—possibly built by the Jewish troops that controlled the Herodium during the Second Revolt (ca. AD 132-135).
This staircase led up to the top of the "mountain" and was the formal entry into the upper palace. It was evidently built in two phases: arches below the existing staircase are from the first phase of the staircase. It seems that the second (final) phase was constructed in anticipation of the visit of Marcus Agrippa, Augustus's right-hand man, in 15 or 14 BC.
After that visit, Herod began constructing his mausoleum to the east of this staircase. The staircase itself was covered over when the whole "mountain" was covered with fill that gave the "mountain" its distinctive volcanic shape—as a memorial mound. Thus, the staircase and the entrance at the top of it are very well preserved.
Josephus wrote in his Antiquities that ". . . it has a steep ascent formed of two hundred steps of hewn stone" and in War ". . . and provided an easy ascent by two hundred steps of the purest white marble."