Stronghold was a term used to indicate a well protected structure.
Vulcans's T'Karath Sanctuary was a stronghold during a Vulcan civil war. (TNG: "Gambit, Part II")
Elim Garak once called Deep Space 9 "a stronghold of his people's most hated enemies". (DS9: "Call to Arms")
In late 2371, the former members of the Shakaar resistance cell headed to a stronghold on Kola Mountain. (DS9: "Shakaar")