Impregnable means incapable of being taken by assault. Although they tried to attack the castle, the enemies found our castle walls to be impregnable, so they stopped their attack before nightfall.
Made out of concrete and lead, the fallout shelter seemed impregnable. They built stout castles in the middle ages believing them impregnable, until gunpowder came into use.
When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).