Smithsonian Magazine published an interesting story (``Riddles of the Anasazi,'' by David Roberts) about the ancient Anasazi of the Four Corners region of the U.S. southwest. For centuries they built buildings and roads and practiced agriculture and pottery. Then around the year 1200 they started suffering depradations by parties as yet unidentified who attacked them, killed them, and ate them; the evidence of cannibalism has become hard to refute.
At first the Anasazi reacted by building residences in increasingly hard-to-reach niches in cliffs. The example on the left is one of the last things they built on the cliffs; a wooden platform wedged into a rock face.
Eventually, at the end of the thirteenth century, the Anasazi abandoned their cliiff faces and moved to mesa tops to the southeast. At least three mesas, each of which could see at least one of the others.
``It was not difficulty of access that protected the settlements (none of the scrambles we performed here began to compare with the climbs we made in the Utah canyons), but an alliance based on visibility. If one village was under attack, it could send signals to its allies on the other mesas.''
The mesas did have perimeter defenses: they were 500 to 1000 feet tall, and they each had only one way in. But their individual perimeter defenses were not as extreme as back on the cliffs, and perimeters were only part of the new mesa defense system. Their descendants the Hopis still live on mesa tops.
Related to the question of Forts vs. Spimes, in this case ever more restricted fort perimeters did not work. What apparently did work was coordinated observations and cooperation. The analogy to the Internet probably does not need belaboring.