IMPREGNABLE

[adjective] - (of a building) so strong and well-made that it cannot be broken into or taken by force, or (figurative esp. UK) (of a person or team) powerful and not likely to be beatenYou can fit your house with burglar alarms and double locks but your home will never be impregnable against determined thieves.(figurative UK and ANZ) Three championship titles in one year has put this top-class golfer in an impregnable position.