Impregnable

adj.resilient, capableformal

Meaning 1

So strongly protected that attack, entry, or defeat appears impossible.

Reach for it when using forceful praise for a defense or position that has resisted serious challenge

The hilltop storehouse seemed impregnable behind its layered stone walls.

everyday
secure
reasonable protection is the honest practical claim
not when Secure is deliberately weaker and better when no absolute test exists.
professional
fortified
deliberate defenses have been added
not when Fortified describes preparation and stops short of claiming those defenses cannot fail.
professional
impenetrable
entry through a boundary is physically blocked
not when Impenetrable may mean difficult to understand rather than secure.
formal
invulnerable
exposure to harm appears absent
not when Invulnerable can describe a person or system and is less tied to defenses.

Unable to be successfully challenged or refuted.

Reach for it when describing an argument whose evidence closes every material gap

Independent records made the chronology nearly impregnable.

playful
watertight
informal praise should stress that an argument has no obvious leak
not when Watertight sounds casual and should not overstate evidence with unresolved limits.
formal
unassailable
criticism or attack cannot find a viable opening
not when Unassailable fits claims and reputations better than a physical enclosure.

impregnable fortress
The ridge made the fortress appear impregnable from the valley.
seem impregnable
No network should be assumed to seem impregnable after one audit.
impregnable argument
The dated records gave her an impregnable argument for preservation.

Impregnable is an absolute claim. Practical security writing should prefer a bounded tested claim unless failure has genuinely been ruled out.

The definition and the example on this page are written here. The dictionaries above ground which meaning is being described, not the wording.