释义
n.
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。
〈喻〉 不受外界影响（或干扰）的人（或事）
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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
