zhè / zuò / chéng / chí / gù / ruò / jīn / tāng / dí / jūn / shǐ / zhōng / wú / fǎ / gōng / rù

这座城池固若金汤，敌军始终无法攻入。

The fortress is virtually impregnable, and the enemy has never been able to break in.