In the Middle Ages, the strength of a castle lay in the width of its walls. In the fortified castle of Sedan, the thickness of the walls varies from 7 to 26 metres. Consistently strengthened in response to advances in gunpowder artillery, the fortress proved to be impregnable. Its surface area of 35,000 m² distributed over seven floors makes it the largest medieval fortress in Europe. The principality of Sedan comprised twenty-one villages whose inhabitants could take refuge in the castle in case of invasion. The motto of the princes, “Undique Robur” (“Strength from all sides”), was adopted by the City of Sedan in the 20th century.