名词
中文翻译：城市防御工事
同义词：
['walled city', 'moat']
例句：
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1. The ancient city was surrounded by a deep moat.
这个古城被一条深深的护城河包围。
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2. The soldiers were guarding the walled city against an attack.
士兵们守卫着这座城墙，抵御着攻击。
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3. The ancient city was built with a moat to protect it from invaders.
这座古城建有护城河，以保护它免受侵略者的攻击。