In fantasy and in other genres the castle is something armies attack. Occasionally a band of plucky heroes sneak in before the army arrives, but as a general rule if it has crenellations and a porter its purpose in life is to be besieged and preferably beaten in some great battle.
Nothing could be further from the truth.
A castle has two jobs: to exert authority, and survive. To exert authority it needs to have a minimum compliment of troops and a significant compliment of administrators. To survive it needs more castles and a bit of luck.
When William the Conqueror swept England after the great battles of 1066 one of his first priorities was to ensure motte and bailey castles went up as quickly as the Normans could build them, wherever they could build them. There wasn't a central planning bureau picking out exactly where things should be placed; put walls up, put archers on those walls, and send out the tax collectors, was the sum of the Conqueror's plan.
When Rome sent its soldiers out into the world and beat seven bells out of whoever it might be this week, its armies built fortresses or castrum as quickly as possible. Some permanent, some temporary, but all with the same purpose: plant the standard and provide a headquarters for the administrators. Possibly also a marketplace to do business with the locals, if circumstances warranted, but that wasn't its main purpose. Its reach extended further than its walls; it controlled a vast swathe of territory.
Look from culture to culture, from Sengoku Japan to the American frontier or the Aztec empire, and you'll find the same. First there is the period of conquest, where simple forts are flung up as quickly as possible. Not all of them will be built in good locations, but that doesn't matter. What matters is that they fulfil their first purpose: to exert authority. In time the castles that no longer matter or which were built in poor locations will be abandoned. Sometimes this means they'll be destroyed so someone else can't use them, and at other times or in less organized societies old forts will be left to decay. Meanwhile the better designed or situated castles will be improved upon. Each surviving castle will carry out its intended purpose: to provide a base of operations for administrators to collect taxes and organize civil operations, say a court of justice.
However survival depends not on defenses but on your neighbors, and that's why castles need more castles.
Back in the day if you were standing at the top of a decent-sized tower and looked in any direction, odds were you'd see another castle off in the distance. It might only be a speck on the horizon, but it's there. The reason why it's there is very simple: an attacker might get one, but not all.
If you're attempting to invade a country your job is to get from the entry point to the main target as quickly as possible. This is even more true in situations where your army has to move on foot through difficult terrain, as has been the case for most of human history. There are only so many months in the year. Some are rainy, some are frozen, some are too hot. Your army might only have three or four months before it has to dig in and wait out the weather. That assumes you have a cohesive, professional army that is paid and trained to fight. For much of history that simply wasn't so. Generals and Kings had to make do with what they could scrape together, but that meant conscripting farmers and other people whose absence was keenly felt at home. Farmers have a nasty habit of deserting when the harvest and planting seasons come. Sailors don't want to work in dangerous conditions for little pay when they could earn much more in the merchant marine.
Even in the present it's ruinously expensive to keep a professional army in the field for longer than a few months. Remember a while back when I talked about the Muscle in Night's Black Agents? I quoted an article posted in the Guardian which said child soldiers fresh from conflicts in Africa were being hired by mercenary companies like Aegis - formerly Sandline - to do duty in Iraq.
"You probably would have a better force if you recruited entirely from the Midlands of England," [James] Ellery, a former brigadier in the British Army [and former director of Aegis], told the Guardian. "But it can't be afforded. So you go from the Midlands of England to Nepalese etc etc, Asians, and then at some point you say I'm afraid all we can afford now is Africans."
The point being that if your job is to get to and conquer the main target, anything that delays this advance is a threat to your economy as well as your campaign. If there are twenty castles in your way then there are twenty threats, and there are only so many options open to you.
You can attack. Congratulations, you just lost the war. A siege takes weeks, perhaps months, and may require specialized equipment you don't have. One siege is doable, maybe, if you're lucky. Twenty is not an option. Even if you capture one or two by quick movement and surprise attack, the others will be waiting for you.
You can ignore them. Congratulations, you just lost the war. Each of those castles has a defensive force and now that defensive force is free to attack your supply lines, your stragglers, and anything else it can get its grubby little hands on.
You can devote a small portion of your force to keep each castle's defenders under siege while moving your main force to the primary objective. Congratulations, you just made things much, much harder for yourself and may have lost the war. Your army is only so large. There's only so much it can do, and if you send one force off here and another off there and a third to God alone knows where, by the time you get to the thing you actually want to attack you may not have enough troops to knock it over.
This is why some of the most successful military escapades in medieval history were hit-and-run raids. The Hundred Years War and Omar's relentless attacks on Marlo Stansfield have this in common: it's not about territory, it's about making the other side bleed. Send the Black Prince and a band of thugs on Chevauchee, pillage everything in sight and run away before the enemy can get an army together. Do that often enough and you put the enemy on the defensive, forcing them to meet you on terms of your choosing.
This is also why when castles fall it typically isn't to an attacking enemy. When castles fall, it's because someone betrayed them. It might be that the defenders reached an accommodation with the attackers before the fight even began, or it might be that one or two people inside the castle let the enemy in. There's many a ballad and ghost story about a foolish lover who let her paramour into the castle, only to be stabbed and left in the ruins as the besiegers take the fortress.
All that said, what does this mean to your RPG campaign?
It means that when you design a castle in your game you need to think about its jobs. It has to exert authority and survive. Authority doesn't mean knights and heroes, it means soldiers and administrators. Someone has to collect the taxes, administer justice, and do all the pesky things that need doing if the country's going to run smoothly.
It's great if the guy in charge is a hero, but it's not about the heroism nor is it about the noble lineage. When William divided up England he didn't give out manors to the most blue-blooded Normans he could find - he was William the Bastard, after all. He gave out manors to successful soldiers, and if, as time went on, those soldiers proved unworthy or died childless the manor would be handed out to someone else. Manors change hands all the time. That's what makes them so useful; the more manors you have in your gift, the more loyalty you can purchase from would-be manor holders.
In a fantasy setting, or in any setting where mundane power - swords, guns - can be outclassed by magical or supernatural power, this does not change the castle's first job. Its first job is still to exert authority. However it does change the castle's second job, because survival in a magical world is tricky business.
The point behind having a lot of castles is that it eats up an attacker's time. An army can't afford to lose even a day, never mind several weeks or months. However if an army has a dragon on its side then a castle might fall in a day. In fact, a dragon might fly ahead of an army and take castles by surprise, forcing their immediate capitulation. An undead army might march more quickly than a living one - no need to worry about fatigue - and doesn't care whether it's winter or summer. The list goes on.
So castles will need some form of defense against the threats that face it, and that may change the way castles are designed. When the threat was that someone might mine under the square corner of a wall to cause it to collapse, castle builders started using round towers which were much less vulnerable to that technique. When cannons made tall walls obsolete forts lowered their profile, eschewing tall walls for long, low palisades with interlocking fields of fire, to keep the enemy from getting too close.
If the castle designers know that dragons are going to be a threat, they'll start putting in anti-dragon measures. If they know undead armies are a thing, they'll start putting out anti-undead measures. Exactly what those are will depend on the enemies' characteristics and power set.
Say this is the typical kind of undead we're talking about, that can be defeated by holy power. In that case every castle has a picket line of shrines, each with its own minor holy relic - something blessed by a high-ranking official, a saint's finger-bone, what-have-you. The point being to keep the undead from getting too close, as well as providing a kind of early warning system. There will be at least one person at the castle whose sensitivity to the ebbs and flows of magic warns them when one of those shrines is interfered with. Equally it will be someone's job to make sure all those shrines are well maintained.
Moreover it will be someone else's job to steal the holy relics, because every castle needs them and there aren't enough to go around. There will be a secondary market in relics, many fake, because when demand is high and supply low, or restricted, black markets spring up like mushrooms. So it will be someone else's job to detect fakes, and prosecute the people stealing holy relics. Or just kill them. Whichever works.
This applies across the board. Castles aren't just for fantasy systems after all; every setting, from science fiction to gritty noir, has its equivalent.
Say we weren't talking about castles. Say we were talking about Facilities in a modern day game. Does anything change?
Not really. Whether a Facility is dedicated to Manufacture, Collection, Distribution, or Analysis, it still has those two basic functions. It has to exert authority and it has to survive. Exactly how it goes about those two functions will change depending on the setting.
Let's say this is an extra-legal facility in a setting like Night's Black Agents. Lets say that its job is to distribute McGuffins, and that it has to do that job within a developed, moderately policed environment. Anywhere in Europe, really; somewhere there's plenty of communication links, transport links, free movement, and enough cops to keep the peace. Not where movement is restricted or there's a significant number of secret police or armed forces checking everyone's papers.
So here is your castle: it's one distribution facility. It has to exert authority, and it has to survive. It has to do those things in an environment that is innately hostile: if the cops knew it existed, they'd shut it down, because it's an extra-legal facility.
So how does it exert authority and survive?
Castles exert authority by providing a base for administrators and tax collectors. The same applies here. The collection authority is a base for, say, vampire Conspiracy types. Since it's a distribution facility it probably doesn't have troops and bosses; it has trucks, drivers, and a bunch of goons. As D'Angelo points out in the Wire, the drug stash never moves without soldiers to protect it.
The exact nature of that authority will depend on the nature of the facility. A low-level, unimportant facility probably doesn't have powerful soldiers, and so on. But powerful or not, someone at that facility is in charge of making sure everything works smoothly, that the McGuffins get to where they need to go when they need to get there, and that the police or whoever is nominally in charge doesn't interfere with the facility's operation. Those are the administrators.
It also has to survive. Castles survive because there are lots of castles. The more of them there are, the less likely it is that any attacker will be able to take them all on. Sure, one or two may fall, but there are always other castles. The sheer number of them may be enough to persuade enemies not to attack at all.
Facilities can use the same trick. There's never just one distribution facility. There are dozens, hundreds. The larger the organization, or Conspiracy, the more likely it is that it will have many facilities at its disposal, some more important than others. If one gets destroyed, move operations to another.
There can be alternate means of survival depending on the nature of the threat, just as the best defense against an undead army isn't necessarily a large number of castles. If the threat comes from a bunch of unaligned burnt spies, then one way to ensure survival is to turn the Heat up as soon as the agents show their faces. Squeal loudly and often; let the cops know that crazy gun-toting madmen are on the prowl. Or, if the facility can't afford to get the authorities involved, tell other criminals. The agents aren't the only one with the Yojimbo option. "You know that shipment you were supposed to get, but which was intercepted by the cops? Yeah, it was these guys." For 'intercepted by the cops' read 'we deliberately didn't sent it' or 'we set it up to look like an ambush.' Whichever best suits the situation.
That's it for this week. Enjoy!