In Tamid 25b, at the end of the explanation for the term "היו עליות", Rashi writes that no one, except from members of the House of David, could sit down in the Azarah. The Cohanim stood "above" Beit haNitzotz and could rest there, since the higher floors where not sanctified. However we know, from the same massechet and Middot 1:6, that part of the guarding of the Temple was done by Leviim, who stood "below". Does that mean that Leviim could not sit down during their guarding service?
Tamid 27a states explicitly that the leviim guarded on the outside so they could sit down.
The Ra’avad at the beginning of Tamid says that for the sake of guarding the Beis Hamikdash they were allowed to sit in the Temple courtyard.