Impossible or very difficult to capture or conquer; impregnable (often used for fortresses, defenses, or metaphorically).
難攻不落の城を築くために、城壁をさらに高くした。
They raised the castle walls further to build an impregnable fortress.
彼のチームは難攻不落で、最後まで得点を許さなかった。
His team was impregnable and didn't allow any points until the end.
新しいセキュリティ対策でシステムは難攻不落になったと会社は主張した。
The company claimed that the system had become impregnable with the new security measures.