Walls defended cities for millennia against attacks from invading armies. Their worth was proven time and again. It was safer for the defendant to stay locked indoors, with food and water in stock, and weather it out till the resources of the attacker were depleted. Siege warfare was won by patience and a strategy of attrition; as well as by sound wall engineering and smart allocation of defending resources. From the fourth century till the beginning of Renaissance the glorious city of Constantinople was defended successfully against waves upon waves of foreign invasions from all directions thanks to one of the finest and strongest walls ever built. But on 29th May 1453 the Walls of Constantinople came tumbling down, and along with them came the crushing end of the Eastern Roman Empire. The Ottoman conquerors had destroyed the city walls using a newly invented technology: the cannon.

Source: The Walls of Constantinople and the new era of cyberwarfare