On one level, medieval castles don’t make sense. They’re imposing, sure. But can’t an invading army just march right past them, laughing all the way?
Castles are not a fully fortified frontier, like the trench lines in World War I. They’re not a walled city that encloses territory that is crucial in itself. Those types of fortifications entirely prevent access to areas of value unless they are breached.
As a recent trip to Aquitaine brought home to me, medieval castles are imposing but not very large compared to the countryside around them.
The castle dominates the ground on which it stands, but it doesn’t actually stand on that much ground.
So are they pointless?
No. There are four main reasons that medieval castles had military value.
1. Dominating Lines of Communication
Castles are often placed along key transportation routes that an invading army wants to keep open to stay supplied.
That’s the case for the two castles I visited in Aquitaine, both of which dominate the Dordogne river.
An invading army can bypass the castle, but it will have to contend with its supply lines being interrupted by the soldiers inside the castle.
After all, an invading army also only dominates the ground on which it stands. Once it moves on, the troops in the castle dominate the area around the castle.
Will the crucial barge of food make it down the river? Cross your fingers!
2. Attrition, Friction, and Delay
An invading army with a castle in its rear has unappealing options. One is to abandon its supply lines and live off the land.
The trouble is that the countryside only has so much food in any one place. Living off the land means spreading the invading army out so it can survive on what it can gather.
An invading army that’s spread out can’t mass its combat power in any one place. That stops it from concentrating against enemy cities or armies.
The whole point of sending a large invading army was to have concentrated combat power!
Another option is to mask or besiege the fortress. Both of these involve putting troops around the castle to encircle it.
Masking means having enough soldiers to prevent the troops inside the castle from getting out and interrupting the line of communications that the castle dominates. Besieging means devoting an even greater number of soldiers to ensure that no supplies can get into the castle so that the troops inside ultimately starve and surrender.
The trouble is that both of these missions require a lot of time and a lot of soldiers. Masking forces have to be much larger than the number of troops in the castle because they have to establish superiority at every point around the castle.
If they don’t, the soldiers inside the castle will sally out and attack the masking force whenever and wherever they spot weakness. The roles of attacker and defender can change in an instant at the tactical level – surprise!
A besieging force needs even more troops than a masking force. Resupply efforts may come from different directions and add more defending troops to the mix. A successful siege could take a year or more.
A final option is to take the castle by storm. This requires less time (if it works!!) but requires even more soldiers. At least before the advent of siege artillery, castles were extremely hard to capture (would you want to climb up those walls!?!).
Storming efforts were extremely costly and most failed.
So let’s take storming the castle off the table for a second and consider masking and besieging. Why not just devote the necessary time and troops? You gotta do what you gotta do, right?
The trouble is that delay plays into the defender’s hands. The attacking army has to find food and is losing troops to disease and desertion every day. The defending troops inside the castle have stored food and water to be ready for a siege.
The defending country also has armies outside the castle. While they also face similar challenges, they have an easier time finding food (it’s their country!) and replacing soldiers (ditto).
The attacking army may begin the war with far more soldiers, but that numerical advantage withers if it has to peel troops off to mask or besiege castles and then faces attrition with passing time.
3. The Possibility of Relief
Delay also opens up the possibility that the balance of forces will shift in favor of the defender in a more fundamental way.
One way this happens is obvious: The defender gets its act together and raises a big army to fight off the attack.
The other is more insidious. Governments are fragile things, even under the best circumstances.
Feudal governments most assuredly did not represent the best circumstances! They were bound together with complex personal loyalties that coexisted with ongoing rivalries.
John may be king, but Duke Robert thinks he’d be a much better king and hasn’t forgotten that John’s father killed his father during the last round of civil wars. Duke Robert may be sworn to King John, but Duke Robert has his own feudal army that is ultimately loyal to him.
Now imagine that King John is far away at war and Duke Robert is still at home with his troops. As the war drags on, Duke Robert may see an opportunity!
Long wars create stress on both sides’ political cohesion. But that stress usually impacts the attacker more since the defender’s coalition is bound together by the shared possibility of losing their lands to the attacker.
The longer the castle can hold on, the more likely it is that the attacker’s political coalition breaks, making it impossible for them to sustain the war.
4. Store of Combat Power
Think of a castle like a military bank account. The defender contributes to it in little installments over the years as it builds the castle and then can withdraw the value whenever it’s attacked.
The attacker’s situation is different. It has to pay cash all up front by massing a huge amount of combat power to capture the castle all at one point in time.
Getting combat power together all at one point in time is hard. These days, if we need lots of resources right now for something that will pay off in the future, we take out a loan.
It wasn’t so easy in the medieval era. Banking existed, but it was vastly harder to borrow to finance wars. Finance was less developed and there weren’t as many clear revenue streams to borrow against.
What’s more, markets weren’t very elastic. It was hard for an attacker to throw a bunch of money at the problem and buy the combat power it needed.
Take manpower. Training soldiers took a long time given the prevailing methods. Feudal knights trained for a lifetime. Longbowmen had to practice for years. Feudal societies couldn’t produce many trained troops in a short span of time.
If I showed up with a ton of money and said I was going to war in three months, I’d have a very hard time buying more trained manpower past a certain point.
Training couldn’t be skipped. If I cobbled together a bunch of peasants with pitchforks at short notice, I’d get more mouths to feed but not necessarily more combat power. They’d flee if charged by well-trained cavalry.
This wasn’t strictly a technological limitation. Ancient armies had massed large numbers of trained infantry that could defeat cavalry. They were able to stand their ground.
Those ancient armies used weapons like pikes. A pike is basically a long pointy stick.
You don’t need to channel Aristotle and have a scientific revolution to get masses of infantry with pikes. You just need to be able to call upon, organize, and train a lot of people… and then give them long pointy sticks.
So it wasn’t that feudal armies couldn’t expand because they lacked gadgets. They were limited by a lack of governmental capacity and social organization. Training large numbers of infantry takes a stronger society.
Food is another example of inelasticity. Feudal agriculture didn’t produce very much surplus food. Food was also very hard to transport from one place to another (particularly if a castle blocked the river!).
If I showed up with a ton of money and wanted to buy more food, I’d be able to buy some. But past a certain point, I’d have more and more money chasing less and less available food. Prices would skyrocket.
At some point, I’d be emptying my treasury for a snack.
All the while, the defenders are sitting in their castle paying relatively little in the way of marginal costs for their defense. They’d accumulated combat power over decades and are now reaping the benefits.
Bonus
There’s one other key point for the modern observer to bear in mind: Castles are a great place to contemplate the complexities of history while responsibly enjoying an appropriate beverage!
