The levites guarded the additional 21 stations, in the following locations:
To begin with, five groups of levites stood guard at the five gates leading into the Temple Mount.
Background: The Temple Mount Gates
A brief description of each of these five gates:
The double "Hulda Gates" on the southern side of the Temple Mount. These two gates were called after Hulda the prophetess (see II Kings 22:14), who sat between them during the days of the First Temple, and told her prophecies to the people entering and exiting the Temple. These two gates served as the main entrance and exit for the Temple Mount. Since the majority of the buildings and activities were on the southern side of the complex, most of the traffic was through this side, and therefore two gates were necessary to accomodate the flow of people.
The "Kiponos" Gate on the western side. This gate was probably named after the benefactor who contributed the funds for its construction, although some authorities maintain that the word is derived from the Greek for "garden work," and that a rose garden was located in the proximity of this gate. This gate also facilitated entrance and exit.
The "Tadi" Gate on the north. Unlike the others, this gate was not used for gaining access to and from the Temple; in fact, in the language of the Mishna, "it served no purpose at all." According to some opinions, it was built exclusively for decorative purposes, and added in beautifying the edifice of the Temple. But other scholars maintain that a priest who inadvertently became defiled (and thus, would have to cease his service, and exit the holy areas of the Temple in order to purify himself by immersion) would exit the Temple unobtrusively through the Tadi gate (as indicated by the Aramaic translation of the word, which carries a connotation of "modesty" or "secret"). There are other possible explanations for this name as well.
The "Eastern" Gate, located on the east side of the Temple Mount. This gate featured a massive illustration of Shushan, capital of ancient Persia, which was created in honor of the Persian kingdom that ruled in the Land of Israel during the first days of the Second Temple (having granted permission for the rebuilding of the Temple). It was through this gate that the High Priest would escort the red heifer and the entire entourage of priests and assistants to the "Mount of Annointment" (the Mount of Olives) to the place where it was burned for the process of ritual purification.