Fortified place (7)
(Other definitions for garrison that I've seen before include "Troops stationed in a town or fortress" , "Military post where troops are stationed" , "HQ" , "Roarings (anag.)" , "Fortified troop station" .)
'fortified place' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bastion that I've seen before include "A projecting fortification to increase angle of fire" , "staunch supporter" , "'Stronghold, citadel (7)'" , "part of stronghold" , "Strong defender" .)
(Other definitions for stronghold that I've seen before include "Defence" , "Fortress" , "castle" , "Keep" .)