Fortified place (8)
(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" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fortified place</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)