fortress
/ˈfɔːtrɪs/noun
1- a military stronghold, especially a strongly fortified town fit for a large garrison要塞; 堡垒。
1.1
- a heavily protected and impenetrable building堡垒般的建筑。
1.2
- 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。
词源
Middle English: from Old French forteresse 'strong place', based on Latin fortis 'strong'.