Advantages of building many castles in the High Middle Ages

Why did medieval rulers like Richard the Lionheart or Edward I build so many castles? Castles were very expensive, especially given poor tax-collection infrastructure; yet the historical record shows that successful conquerors expended great effort (and revenue) on the construction of numerous fortified stone castles. They embarked on these programs of mass construction not out of ignorance but, instead, because of economic and strategic considerations that made efficient use of castles a vital part of conquering and holding territory.

I will briefly summarize the description of castle-building in the High Middle Ages given by Castles, Battles, and Bombs: How Economics Explains Military History, drawing freely on excerpts from the section The Case of the Medieval Castle and the Opportunity Cost of Warfare (text freely available at the link).

Summary of the benefits vs. drawbacks of castles

There are many reasons why one might initially refrain from building castles:

  • Castles are expensive (mostly from the labor cost)
  • If you give a castle to a lord, the lord can use it to turn against you
  • Money spent on castles is money not spent on a field army
  • Castles are immobile

These are quite compelling, and one might indeed wonder why medieval kings built so many castles all over Europe. They appear to have done so because of the following advantages:

  • Medieval supply chains were weak, so being in proximity of a friendly castle is an enormous advantage
  • It was very expensive to take an opposing castle, requiring huge investment into a storming force or a prolonged siege
  • Castles serve as political symbols, reminding surrounding townsfolk of their ruler

Imagine that you are trying to take some unfriendly territory. The enemy castle is a reserve of fresh troops, food, water, medicine, etc., which allows them to constantly raid your forces and retreat back into the castle at the end of the day. To actually take the territory, you need to conquer the castle, requiring that you either storm the castle (with at least 4x and up to 50x as many troops as reside within the castle) or siege the castle until they surrender (which is incredibly expensive for you, given that you may be far away from essential resources). The cost-effectiveness of castles is readily apparent.

When you have conquered territory, too, it is not difficult to see how building a castle allows you to maintain local control. You can raid nearby territory for food, currency, and so on, and store your plunders inside the castle. The castle then also serves as a “stepping-stone” that extends the range of your supply-chain network. As such, the castle is crucial in both offensive and defensive capacities.

In general, understanding the important role of castles in medieval warfare yields a much better appreciation of the overall nature of medieval combat and life in the High Middle Ages. The projection of force was extremely expensive due to the difficulty of obtaining, transporting, and delivering supplies; in this setting, dotting the landscape with fortified bases where you can build up spare capacity from a strong defensive position is a very natural decision.

Choice excerpts

On the difficulty of storming or besieging castles:

The attacker faced severe costs. Money supplies sometimes succumbed before the defense did. Stephen (r. 1135–1154) spent £10,000 besieging Exeter Castle, a sum five times his estimated annual income. Indeed, a successful siege could be financially disastrous. Henry III (r. 1216–1272) captured Kenilworth in 1266 through negotiation, “but the siege had been cripplingly expensive, absorbing the income of ten English counties.” The personnel cost constituted the real culprit. On both sides specialists were needed. A long siege quickly raised expenses because feudal levies could only be required to serve for periods set by tradition and law, and once these were exhausted, they had to be paid. The cost of personnel—to ruler and soldier—was high. Estimates of the numbers needed for a successful storming range from 4:1 to 10:1, and this seems conservative in view of some castles’ successful defense against odds of 50:1. For the soldier, the attack was extremely hazardous and even after a breach of the wall it was not always easy to get troops to assault, sometimes requiring leading nobles and rulers to lead from the front. This could easily backfire. In 1088 a king was leading an attack on a gate when a woman described as “female in sex but not in spirit” threw a millstone on his head, bringing his reign to a premature end.

On the inherently defensively-favored nature of medieval combat:

In an age where capacity to stay in the field was limited, this gave the castle a crucial advantage. The historical literature is filled with examples of medieval commanders avoiding battles, and some never fought any at all. With a castle as a handy retreat, there was no need to fight unless victory was certain, in which case the adversary would not fight. Clausewitz’s dictum that “no battle can take place unless by mutual consent” is unusually accurate for this age. Most invaders used a single line of advance and could easily be evaded. When Edward III (r. 1327–1377) invaded Scotland, the Scots either avoided battle or took up strong defensive positions the king was unwilling to assault. Even when multiple lines of advance made complete avoidance impossible, the weaker force could retreat into its castles and wait for the fighting season to end. Fulk Nerra avoided battle whenever possible. Even when it was not clear that the enemy was stronger, fighting from behind walls was tempting.

On the benefits of building castles as part of an offensive push:

Rarely was the defense of a castle a passive operation. “The overriding thought in castle strategy was not passive defence but action and destruction.” Moving across the drawbridge actually strengthened the “retreating” force. “Shutting oneself up in a castle was not an attempt to avoid conflict, but a manouevre to make the enemy fight at a disadvantage. . . . The defence had an enormous advantage. To an invader time would be vital.” Depending on distance, essentially three cases existed in which the castle garrison threatened an enemy. First, sallies might be made against the immediate besiegers. Second, raids from the castle could threaten areas within a day’s round trip or so.

On the resolution of combat through surrender rather than direct battle:

No definitive numbers exist for successful versus unsuccessful sieges. That the failure rate was very high is undisputed; indeed, the majority of successes were the result of negotiation, not storming. William the Conqueror allegedly never failed to take a castle, but he did so mostly by agreement. Even after a three-year siege of a castle in Normandy, the famous duke offered terms because he “had been forced to recognize that taking the castle was too demanding in time and resources.”

On the labor required to build fortified castles:

In the days of the motte and bailey castle, the labor required for building a very basic fort was small. A hundred men working for a month could build a modest one, and if they labored about three months, a substantial mound could be constructed. The stone castle was another matter. The tower constructed at Langeais required about 83,000 “average work days” to construct, ten times what a substantial motte and bailey needed. Langeais was built in two seasons. If the workforce was able to labor six months each year, there would have had to be some three hundred laborers continuously on site. The labor of an additional 1,000 to 1,200 farmworkers would have been needed to sustain the builders.

Edward I’s castles required much more (fig. 2.1). None of his fortifications were completed in less than five building seasons (six to seven months each), and three took much longer. Many of the works were done at the same time. After 1277, Builth, Aberystwyth, Flint, and Rhuddlan were being built at the same time; after 1283, Conwy, Caernarfon, and Harlech were going up together. The number of workers at each castle varied enormously. At Builth only a hundred might be present at any one time, but at Beaumaris the total exceeded 3,000. In 1283–1284, about 4,000 men were employed at Conwy, Caernarfon, and Harlech, while 3,500 were hired in 1295 to work on Beaumaris and repair Caernarfon (which was damaged in an uprising). The population in Edward I’s day was about three to four million people, so that the use of 4,000 workers was not a negligible number (an equivalent number of Americans today would be about 300,000).

On the eventual decline of castle-building as a natural complement to the rise of strong, centralized nation-states:

Whether the castle could have survived much longer once gunpowder became prevalent is actually a moot point. Its very strength was a barrier to the growth of great national governments. Monarchs regarded private castles as an inherent threat. Legal and other measures were taken to eliminate them. In Britain, the Tudors were particularly effective in eliminating great noble castles as part of a well designed program to establish the state’s monopoly on violence. In France, Louis XIII probably destroyed more castles than he built. This trend was actually a paean to the military virtues of the castle.


Hocwyn Tipwex on Twitter adds:

It was also part of a counter trend after a period of extreme decentralization to the growth of regionally central market towns. Due to extreme risks+costs of overland transport until the late 18th/e 19th, most value of land was in its proximity to towns

So creating a new permanently fortified town makes a lot of sense when you are moving to more advanced cultivation techniques including capital-intensive ones like windmills, water mills and light manufacturing

January 28th, 2023 | Posted in History

One Response to “Advantages of building many castles in the High Middle Ages”

  1. saila Says:

    Obviously there are significant path-dependencies but exploring the game-theory behind historical development is always interesting — I’ve read a lot about sports with regards to this.