impregnable

An impregnable place or thing is one that can't be broken into or overcome easily. A strong fortress could be an example of an impregnable location.

Think about defending your favorite video game character against a tough enemy - you might want to take shelter behind something impregnable, like a shield or a safe zone. In real life, a city with high walls and deep moats could be considered impregnable until someone invents a way to breach those defenses. The term can also apply to abstract ideas, like an argument that's too complex for someone to refute.