An impregnable fortress or city; a place with extremely strong, unassailable defenses.
这座山顶堡垒易守难攻，真可谓石城汤池，即使强大的对手来犯也难以撼动。
This mountain-top fortress is easy to defend and difficult to attack, truly an impregnable stronghold; even a strong opponent would find it hard to shake.
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