zhè / zuò / shān / dì / xíng / xiǎn / yào / zì / gǔ / yǐ / lái / bèi / shì / wéi / tiān / rán / de / tiān / xiǎn

这座山地形险要，自古以来被视为天然的天险。

This mountain has treacherous terrain and has been regarded as a natural stronghold since ancient times.