Fort Knox was built to be an impregnable fortress.

诺克斯堡被建造成一座坚不可摧的堡垒。

The bullion depository at Fort Knox was built to be an " impregnable fortress."

诺克斯堡的金银存放处被建造成一座“坚不可摧的堡垒” 。

The keep's formidable walls and defensive features make it a virtually impregnable fortress.

主城堡坚固的城墙和防御设施使它成为一座几乎坚不可摧的堡垒。

" Harry - we've just got to grit our teeth and do it, " said Ron on Friday morning, in a tone that suggested they were planning the storming of an impregnable fortress.

“哈利——我们必须咬着牙豁出去了。”星期五的时候，罗恩说道，听他的口气就好像他们正在计划攻破一座固若金汤的要塞。