impregnable

这座城堡的城墙坚不可摧，从未被敌军攻破过。

The castle's walls were ___impregnable and never breached by enemy forces.

他的理论建立在如此坚实的证据基础上，简直无懈可击。

His theory was built on such solid evidence that it became ___impregnable.

她带着不可撼动的领先优势进入最后一轮，只需要再得一分就能获胜。

She entered the final round with an ___impregnable lead, needing only a single point to win.