The fort's defenses were thought to be 'impenetrable'.
an 'impenetrable' wall/barrier/jungle
an 'impenetrable' secret code
(..) Isn't the museum worried that someone might steal it? The Pink Panther is protected. And by forces more impenetrable than any army. Observe. Deactivate. Failing the radar shield, the slightest release of pressure in the weight of the diamond... (..)1975 The Return of the Pink Panther