In the Middle Ages, the strength of a castle lay in the width of its walls. In Sedan, the walls of the castle vary in thickness ranging 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 21 villages, whose inhabitants would take refuge in the castle in the event of an invasion. The motto of the princes, “Undique Robur” (strength from all sides), was adopted by the municipality of Sedan in the 20th century.