The fortress is impenetrable, so it cannot be taken.
{s} incapable of being penetrated or entered; unsusceptible to outside ideas or influences; unfathomable, inscrutable
emphasis If you describe something such as a book or a theory as impenetrable, you are emphasizing that it is impossible or very difficult to understand. His books are notoriously impenetrable. = incomprehensible + impenetrably im·pen·etrably impenetrably detailed reports on product sales. = incomprehensibly
Having the property of preventing any other substance from occupying the same space at the same time
unimpressible; not to be moved by arguments or motives; as, an impenetrable mind, or heart
not admitting of penetration or passage into or through; "an impenetrable fortress"; "impenetrable rain forests"
If you describe something such as a barrier or a forest as impenetrable, you mean that it is impossible or very difficult to get through. the Caucasus range, an almost impenetrable barrier between Europe and Asia
permitting little if any light to pass through because of denseness of matter; "dense smoke"; "heavy fog"; "impenetrable gloom"
not admitting of penetration or passage into or through; "an impenetrable fortress"; "impenetrable rain forests
Incapable of being penetrated or pierced; not admitting the passage of other bodies; not to be entered; impervious; as, an impenetrable shield
impossible to understand; "impenetrable jargon" not admitting of penetration or passage into or through; "an impenetrable fortress"; "impenetrable rain forests