They could have withdrawn into the mountain fastness of Eryri.

他们本来可以撤回到艾瑞的山区要塞里。

《柯林斯英汉双解大词典》

That mountain fastness is quite famous.

那座山寨很有名。

youdao

The bandits withdrew to their mountain fastness for the winter.

盗匪们撤回到他们的山寨中准备过冬。

youdao

Indeed, diplomats who have visited them in the remote mountain fastness of their new capital, Naypyidaw, say they are even more confident.

ECONOMIST: The hopes kindled by the saffron revolution have faded fast