While "impregnable" is a bold claim, Krak des Chevaliers in modern-day Syria is widely cited by historians and architects as the most formidable fortress ever built. Constructed by the Hospitaller Knights during the Crusades (12th-13th century), it features a "concentric" design with two massive stone curtain walls separated by a steep moat. Its walls were up to 100 feet thick in places, and its internal granaries and water cisterns allowed a garrison of 2,000 men to survive a siege for up to five years. Throughout its history, it was never taken by force in a frontal assault; even the great Saladin turned away from its walls after recognizing its defensive perfection. It only eventually fell to the Mamluks in 1271 because of a forged letter from the knights' commander instructing them to surrender. Another contender is The Fortress of Gibraltar, which withstood the "Great Siege" for nearly four years due to its sheer vertical limestone cliffs and miles of internal tunnels, proving that geography is often the best architect for an unshakeable defense.