Stronghold was a term used to indicate a well-protected structure.
(ENT: "Chosen Realm")
(TNG: "The Mind's Eye")
Vulcan's T'Karath Sanctuary was a stronghold during a Vulcan civil war. (TNG: "Gambit, Part II")
(DS9: "Blood Oath")
In late 2371, the former members of the Shakaar resistance cell headed to a stronghold on Kola Mountain. (DS9: "Shakaar")
(VOY: "Basics, Part I")
Elim Garak once called Deep Space 9 "a stronghold of his people's most hated enemies". (DS9: "Call to Arms")
(VOY: "Concerning Flight")
(DIS: "The Sound of Thunder")