固若金汤 as strong as gold and water
Explanation
比喻防御工事非常坚固，难以攻破。
A metaphor for extremely strong defenses that are difficult to break through.
Origin Story
秦末农民起义爆发，陈胜吴广率领起义军攻打各地。其中，武臣攻打范阳，范阳县令徐公坚守城池。蒯通前往劝说徐公投降，并向武臣建议妥善安置徐公，以示宽大，从而瓦解其他守城将领的抵抗意志。武臣采纳了蒯通的建议，果然其他城池望风披靡，纷纷投降。这说明，有时候，攻城略地并不一定需要凭借坚不可摧的武力，策略和智慧也同样重要。当然，如果城池“固若金汤”，任何策略都可能失效，需要强大的军队和相应的策略才能攻克。
During the late Qin Dynasty, peasant uprisings broke out, and Chen Sheng and Wu Guang led the rebel army to attack various places. Among them, Wu Chen attacked Fanyang, and the magistrate Xu Gong defended the city tenaciously. Kui Tong went to persuade Xu Gong to surrender, and suggested to Wu Chen to treat Xu Gong well, showing magnanimity, thereby undermining the resistance of other city commanders. Wu Chen adopted Kui Tong's advice. This illustrates that sometimes military conquest doesn't solely depend on invincible force; strategy and wisdom are also crucial. Naturally, if a city is "impregnable", any strategy might fail; powerful troops and appropriate strategies are needed for conquest.
Usage
形容防御工事非常坚固，难以攻破。多用于军事或比喻意义。
To describe fortifications as extremely strong and difficult to break through. Often used in a military or metaphorical sense.
Examples
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这座城池固若金汤，易守难攻。
zhè zuò chéng chí gù ruò jīn tāng, yì shǒu nán gōng
This city is impregnable.
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他们的防御工事固若金汤，敌人很难攻破。
tā men de fáng yù gōng shì gù ruò jīn tāng, dí rén hěn nán gōng pò
Their defenses were as strong as a fortress of gold and water, nearly impossible to break through.