What does «inexpugnable» mean in Spanish?

That it cannot be conquered by being well defended.

Examples of use in Spanish: "fuerte/fortaleza inexpugnable".

"era un castillo inexpugnable".
Difficult to access or directly inaccessible.

Examples of use in Spanish: "la montaña se volvió inexpugnable tras la gran nevada".

"las inexpugnables profundidades del océano".
Who does not allow himself to be defeated or persuaded.

Examples of use in Spanish: "permaneció inexpugnable ante cualquier argumento".

"era un hombre inexpugnable".

"su inexpugnable obstinación".