The more you learn, the more you discover how little you know. That seems to be the story at Herodium, as the uncovering of a monumental entrance suggests a more complicated building history than previously understood. From a press release of the Hebrew University of Jerusalem:

Archaeologists from The Hebrew University of Jerusalem’s Institute of Archaeology have discovered a monumental entryway to the Herodian Hilltop Palace at the Herodium National Park. The unique complex was uncovered during excavations by The Herodium Expedition in Memory of Ehud Netzer over the past year, as part of a project to develop the site for tourism.
The main feature of the entryway is an impressive corridor with a complex system of arches spanning its width on three separate levels. These arches buttressed the corridor’s massive side-walls, allowing the King and his entourage direct passage into the Palace Courtyard. Thanks to the supporting arches, the 20-meter long and 6-meter wide corridor has been preserved to a height of 20 meters.
The Hebrew University archaeologists — Roi Porat, Yakov Kalman and Rachel Chachy — suggest that the corridor was built as part of Herod’s plan to turn Herodium into a massive artificial volcano-shaped hill, a vast and impressive monument designed to commemorate the architect-King.
Surprisingly, during the course of the excavations, it became evident that the arched corridor was never actually in use, as prior to its completion it became redundant. This appears to have happened when Herod, aware of his impending death, decided to convert the whole hilltop complex into a massive memorial mound, a royal burial monument on an epic scale.
Whatever the case, the corridor was back-filled during the construction of the massive artificial hill at the end of Herod’s reign. The upper section of a new monumental stairway stretching from the hill’s base to its peak, constructed during the course of this building phase, appears to have been built over it.
The excavators point out that not only was the arched corridor covered over in the course of the construction of the hill-monument, but also all the structures earlier built by Herod on the hill’s slopes, including the Royal Theater uncovered by the expedition in 2008, while still led by Prof. Ehud Netzer, since deceased.

The press release continues with more discussion of the site history as well as plans to allow visitors access to all of the new discoveries. Photos are available here.