What is the classical Greek word for "Fortress"?

[Mythos_Ruler]

That's the modern version of the word, while ???????? is the ancient version. From my exposure to Classical and Hellenistic history and epigraphy, I gather that this usually has the meaning of a fort or (by extension) a garrison, but I think it can also mean "citadel" or "fortress." However, in many Greek city-states the citadel was the acropolis, but ????????? had a much more specific meaning and definition (the fortified upper portion of a city which usually served as an important military, administrative, and religious centre).

Thanks, but what is ???????? transliterated into English?