Maimonides states: "The tribe of Levi is completely separated, designated for the service of the Holy Temple; as the verses testify (Deut. 10:8), At that time, G-d designated the tribe of Levi to carry the ark of G-d's covenant, to stand before God and serve Him, and to offer blessing in His name. It is for this reason that Levi was not given any portion or inheritance along with his brethren. G-d is his heritage, as God promised him.'
It is a positive commandment for the Levites to be available and prepared to carry out the service of the Temple, unoccupied with other pursuits... their tasks are divided; some guard over the Temple, some are the gatekeepers, opening the Temple's doors at the start of the daily service, and locking them at the close of the day."
The Bible states, "... and they (the Levites) shall be your associates, and they shall be entrusted with responsibility for the Communion Tent" (Numbers 18:4).
With these words, G-d instructed Aaron in the commandment of establishing watches in the holy place. Unlike ordinary guard stations or "checkpoints," these watches were not for the purpose of guarding against intruders such as enemies or robbers, but rather to glorify the honor and dignity of the House of G-d. If one would not expect to find the palace of an ordinary king or nobleman to be left without an honor guard, how much moreso would such a practice be fitting and proper in the House of the Creator himself, the living G-d of Israel.
Some authorities maintain that these watch stations were manned round the clock, but most others opine that the Temple watches only applied to the night.
Altogether, watches were kept in 24 locations throughout the Holy Temple. The majority of these - 21 - were manned by the Levites, and 3 by priests. Tractates Tamid and Midot enumerate the locations of these various watches throughout the Temple, and most of the information in this chapter is based on information contained therein, and expounded upon by various commentators of the Oral Traditon. It may also be of interest to the reader to note that the scholars of the Temple Institute of Jerusalem have researched and documented the precise location of these stations, using a computerized blueprint of the Temple complex itself.