The city of walls: Contantinople

Atilla The Hun, the destroyer of civilizations, who named himself the Scourge of God, took one look at them and turned around. The Avars battled the walls the uselessly with their catapults until they ran out of rocks. The Turks tried to tunnel under them but found the foundations too solid. The Arabs tried to starve the city into submission but ran out of food themselves and had to resort to cannabalism. It took the guns of the modern world to finally bring them down. In 1453, the Turks brought their super weapon: a monster cannon that can fire a 15 hundred pound stone over a mile.