There's a pop-culture tendency to point at structures like Hadrian's Wall or the Great Wall of China and laugh because "they didn't keep invaders out." But that betrays a very limited understanding of what a wall is for.

Without a wall, anybody can wander through anywhere they like -- terrain permitting, which is why people like to put borders in places where nature itself forms a useful barrier. (Much cheaper that way.) When you build a wall, though, easy passage can only be effected in a limited number of places: specifically, where there are gates. Legitimate traffic will go through those restricted channels, which means that at a minimum, your wall gives you the chance to monitor that traffic. If you want to ask their business, record information, collect taxes, or turn somebody away, a wall makes those tasks much simpler.

Can people get over the wall in non-gate locations? Of course: outside of fantasy, basically no wall is completely unclimbable. But every bit of difficulty you put in an intruder's way is going to limit how easily and, more important, how usefully they can get across. Even a mere palisade of sharpened stakes, like that used to defend the Roman border in Upper Germania, is beneficial in that regard. Sure, somebody can get over it. But can a hundred? A thousand? Without being noticed? Even if they can, their horses sure as hell can't, or their supply train. If they want to bring an effective invasion force through, that small group has to either bring the wall down, or (more likely) hit a gate fort from behind, through a surprise attack or treachery. Then, with the gate in their control, they can actually start the invasion proper.

Defense, however, isn't just about barriers; it's also about surveillance. A wall and its forts make a useful base from which to send out reconnaissance patrols, which might either return word of an approaching army or not return at all -- and that's a warning in its own right. If the defenders are competent, they'll also keep a swath of ground outside the wall clear of trees, so that anybody approaching will be spotted before they reach the wall itself. Once there, ideally no point anywhere along the line will be out of view of a watchtower, even if you have to change their spacing or the path of the wall to arrange that. The result is that even the aforementioned single guy or small force can't go unnoticed, unless they go without torches on a cloudy or moonless night -- which, of course, makes it that much harder to effect a crossing. Once the defenders see anything, they light signal fires or otherwise send an alert, and the larger body of soldiers at a gate fort knows to prepare for trouble.

Nor does it end there! In addition to the watchtowers and forts, a wall frequently has nearby support, in the form of one or more larger settlements with their own garrisons. This place can have support services for the army (you don't want a ton of civilians at your wall), and soldiers can rotate in and out -- wall duty being kind of famously an unpleasant assignment. When something goes down at the border, word also gets sent to the nearby army, which can either ride out in support or batten down the hatches in preparation for an impending attack. This can ripple out as far as it needs to, from that settlement to deeper within the territory, and all the way back to the capital or wherever the ruler happens to be.

In other words, a wall is a larger-scale version of the security principles we talked about in Year Three. To begin with, it serves as a deterrent: attacking someplace guarded by a wall is harder than attacking someplace without, which either diverts the enemy to an easier target or discourages the less well-organized foe. If they attempt something anyway, the wall gives you an opportunity to spot it coming, and to warn others that they're in danger. And finally, it provides a foothold for your response, whether that be killing, capturing, or driving off whoever threatens the wall and everything it protects.

So why don't they always work?

Most failures can be chalked up to an insufficiency of money, of loyalty, or of both. If a state can't or won't pay to properly maintain its wall and associated defenses, then crumbling sections or encroaching forest will make it easier for people to get across unseen. If it can't or won't pay to properly equip, train, and compensate its soldiers, then they'll slack off in their vigilance or be useless when trouble arrives. And poorly paid soldiers -- especially poorly paid commanders -- are more susceptible to bribery. Why bother sneaking a bunch of guys over the wall in pitch-black night and then assaulting a fort when you could just get somebody inside to open the gate for you?

Most of the time, the security failures will be small ones. Somebody takes an unauthorized nap and it's fine, because nine hundred ninety-nine times out of a thousand, nobody's trying to slip across at that exact moment. Guys at a watchtower or two get bribed to look away from, not an invading army, but some smugglers bringing contraband over the border. Maybe twenty guys manage to raid a border village -- and then possibly stay on that side of the wall, marauding through the countryside, because everything they steal makes it that much harder to get back home (assuming they even want to go).

But the big failures are dramatic. Somebody turns coat against their country, maybe for greed, maybe for ideology, but the result is pretty much the same. It may sound like a good idea to get a troublesome general out of your hair by sending him as far from the capital as he can get, but you do risk him deciding he's got better friends on the other side of the wall. If he's competent and ruthless enough, he can keep that warning system from transmitting an alert until his loyalists and new allies are deep into your territory, where there are no more walls to help keep them out.

No, walls don't always work. But when you really need to defend a border, having one is worth the expense. Just make sure you don't stop paying the bills.



(originally posted at Swan Tower: https://is.gd/ZidYV5)