|Roman fort depicted in the Witheridge Historical Archive|
I found it interesting that this feature of Roman military practice was actually adopted from the Greeks. We are told by both Polybius and Livy that both Romans and Greeks used young, preferably oak, trees with three to four substantial side branches. However, the Greeks spaced the trees farther apart, filling in the extra space with more branches. This enabled them to open the palisade by grasping the fill branches and dragging them out of the way, then sally forth from any point of the rampart.
The Romans, however, placed the trees much closer together, interlacing the branches and sharpening the ends into points. It was very difficult to breech and not intended to be a temporary convenience, easily cast aside. Ideologically the Romans used their fortification to reestablish the encircling sacred boundary that was used to demarcate a Roman city from surrounding countryside and once erected remained so until the mobile "city" of the legions moved on.
"Boundaries are very important in the Roman world, and defining those boundaries is essential to knowing what is Roman," explains Professor Steven Tuck, Miami University, in his lecture on Roman military forts and fortifications in his Teaching Company Course, Experiencing Rome: A Visual Exploration of Antiquity's Greatest Empire. He observes that gateways were viewed as a point of weakness, not only from a military viewpoint but from a religious one as a gateway represented a break in the religious protection of the pomerium. "...wherever you put a gate in one of these symbolic religious boundaries, you're creating a zone of religious conflict."
Tuck points out that formal Roman gateways were often protected by a protome, like the bust of Medusa or an important individual, that served an apotropaic purpose to ward off evil.
So we see that, like the eagle standard, the protective palisade served both a military and religious function, protecting the legions while projecting Roman power, reinforcing religious beliefs and demonstrating tangible benefits of Roman citizenship.