名词
中文翻译：堡垒
同义词：
['fortress', 'stronghold', 'stronghold']
例句：
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1.The citadel was once a fortified city.
这座要塞曾经是一个被加固的城市。
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2. The citadel is the most secure part of the castle.
要塞是城堡中最安全的部分。
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3. The citadel was used as a refuge during the siege.
要塞在围城期间用作避难所。