The walls of the castle, sixteen feet thick, were impregnable.
Advanced English Dictionary
+adjective 1 an impregnable building is so strongly built that it cannot be entered by force: an impregnable fortress 2 strong and impossible to defeat or change: The team built up an impregnable 5-1 lead. + the seemingly impregnable Nazi regime
Random quote: If we look at the world with a love of life, the world will reveal its beauty to us.: Daisaku Ikeda