The Blog of Harald

I’ve visited lots of old fortresses in Canada, and a few in Europe, and I remember learning about defense in depth. This is the idea that your assets should be surrounded by multiple separate layers of defenses to make it harder for the barbarian hordes (or Americans :-) to break in. Ideally the defenses should be _different_, so that if a simple technique of defeating one is discovered, it doesn’t help against the others.

The forts I’ve visited are typically on a hill (so that you can see the enemy coming and prepare. But they’re also sunk into the hill, with sloping outer walls, to defend the inner walls against artillery. They’re surrounded by open fields (no trees or brush). The outer wall has gun emplacements, to mow down anyone trying to cross those fields. There is a deep trench between the inner and outer walls, deep enough that attackers must climb down, slowing them down. The inner walls are full of small, narrow windows to allow the defenders to shoot at anyone trying to cross the trench. The inner walls usually have towers that project into the trench, so that people trying to climb the inner walls aren’t hidden from defenders inside the walls; those attackers can be attacked from the towers. The important buildings inside the inner walls have their own defenses. And so on.

Of course, a few carefully placed shells from a battleship and the fort is history; but that’s progress for you.

Anyway, to make a long story short, here’s Robert Scoble’s “defense in depth strategy for Windows XP”:http://radio.weblogs.com/0001011/2004/08/22.html#a8128 – enjoy!