Impregnable

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.

• exposed
• penetrable
• refutable

• 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.