What are/have been the most impregnable fortresses in history? - Quora

What was the most well thought out medieval castle in terms of defense and defender survivability? - Quora

What are some of the best fortified historical fortresses in your opinion? - Quora

What is/was the strongest fort/castle for defense from any era, preferably not English style? - Quora

What were castles built out of? - Quora

What are/have been the most impregnable fortresses in history? - Quora

Are there any European cities with a fortress left? - Quora

What are/have been the most impregnable fortresses in history? - Quora

What was the purpose of building forts around cities in medieval times? - Quora

What is/was the strongest fort/castle for defense from any era, preferably not English style? - Quora

What are some of the best fortified historical fortresses in your opinion? - Quora

Could you dig under the walls of all castles in the Middle Ages, or were there areas where the ground was too hard for that? - Quora