Numbers 18:4 "And they shall keep charge of the Tent of Meeting"

The Beit HaMikdash is so holy that the Torah is not content with most sophisticated safety devices. HaShem commanded the Priests and Levites, the servicemen of the Beit HaMikdash to be "human alarm systems."

The priests keep guard inside while the Levites patrol outside. The presence of these guards is also a means of honoring the Beit HaMikdash.