source: 13th Black Crusade vs Iserlohn fortress | SpaceBattles Forums

a military stronghold, especially a strongly fortified town fit for a large garrison.

要塞, 城塞 (!fortは壁で守られた兵隊の駐屯地; fortressは大規模な城または城郭都市)

a person or thing not susceptible to outside influence or disturbance:

堅固な場所［建物］; (外的影響を受けにくい)心の堅固な人

e.g. he had proved himself to be a fortress of moral rectitude.

Middle English: from Old French forteresse ‘strong place’, based on Latin fortis ‘strong’.