Walls guard. Walls constrain. They defy us to break them. And so it has been since the dawn of agriculture. We see defensive walls protecting ancient cities from China through sub-Saharan Africa. Their widespread prevalence is a testment of their desirability. A wall around a city correlated well with the size of its merchant community, protecting the riches within from the ravages of medieval times while providing an overt display of the city’s wealth, power, and organizational skills.
Over the centuries, attackers kept developing new siege techniques, but the advantage remained with the defender. With every new siege technique, an equally powerful counter measure arose. Succeeding in storming a walled city was beyond the resources of most medieval barons, who tended to limit themselves to castles.
Walls lost their value with the introduction of gun powder. From inside, from the base, or in a cannon, it gave the attackers the advantage. It is an economic argument: a wall is very expensive to build. Before the introduction of gun powder, the pillaging of the countryside or the sacking of the city offered poor incentives to offset the costs of a siege. But building a bigger cannon was much cheaper than erecting a stronger wall. Mehmed had specially built giant cannons to aid his siege of Constantinople.
Modern walls, such as the Berlin Wall or the Peace Lines are used in conjunction with other tools to control populations. The Berlim Wall gave the VoPo enough time to aim and shoot; the Peace Lines make it harder for Catholics or Protestants to wander into each other’s neighborhoods, except through policed gates. Again, the argument is economic: a resource that is scarce (the police) is given a chance to be where needed instead of having to be everywhere. The attackers could easily overcome the wall, but not fast enough to avoid the police.
In the cyber world the walls have worked like the medieval walls of yore. Firewalls keep safe old programs that cannot defend themselves. The DMZs, like the peribolos between the Theodosian walls of Constantinople, hold digital artifacts for inspection by second generation security tools. But like all walls, they have gates through which small things can penetrate, and unlike a medieval city where everyone knew stranger from neighbor, the internal networks have few tools to detect what sneaks through.