a military stronghold, especially a strongly fortified town fit for a large garrison 要塞;堡垒 ■a heavily protected and impenetrable building 堡垒般的建筑 ■(figurative)a person or thing not susceptible to outside influence or disturbance (喻)不受外界影响(或干扰)的人(或事) he had proved himself to be a fortress of moral rectitude. 他已证明自己是坚定不移的公正廉洁之士。
语源
Middle English: from Old French forteresse 'strong place', based on Latin fortis 'strong'