The century of the First Crusade was, to its time, like the first World War to ours. To put it another way, Northern Europe was a “developing nation” while the Near East was modern. The history of medieval Europe is best seen as the story of how a barbaric nation that lived in forests was able to preserve and adapt the best of its values while contact (usually in war) with the East forced it to modernize. What Persia, Constantinople and Baghdad never saw coming was that there would be something in the barbarians’ values that, when combined with machinery and culture, would soon change the game entirely.
When the Franks fought off the invading Saracens at Tours in 732 AD, it was the kind of battle they had been fighting for a long time. True, the invaders were on horses and had slightly different weapons. But the battle was on land, in a forest, and victory depended on slugging it out with axes and spears. The Frankish army’s shield wall, its tried and true war method, held. The invaders had no technology that made the shield wall ineffective.
The First Crusade brought these same methods into a region where war was about Greek fire, siege engines, and massive structures built by slave armies. To defend Constantinople and take Jerusalem, the knights had to figure out how to fight this kind of war. Of course, they learned; they hired experts, tried out experiments, and learned how to win sieges. But the experience was like the first time horse-mounted cavalry went up against tanks.
They were trained in a personal warfare method that inevitably meant champions fighting hand to hand in the middle of a battle, with arrows flying. The new warfare was about stalemate, starvation, and sanitation; it was about hauling huge timbers from distant places and coordinating large teams to operate trebuchets. It meant hiring miners to dig tunnels. Modern warfare was about engineering.
The men who came back from the Near East looked at their Norman castles with very different eyes. Until now, warfare in Europe had all been the Frankish sort. The Norman Conquest, only a generation before the First Crusade, was fought by hand in a day in a field. Those same fighters’ sons looked warily at each other and realized that if someone tried to conquer regionally, it would be with the new warfare. And none of the castles built before 1150 were prepared for it. They were all meant to keep out armies with axes and spears; nobody had used siege engines on these walls.
Castles changed in the late 1100s. Building plans after 1190 were all about siege engines and mining. They didn’t actually get into hardcore war with each other for a while, so they only experienced engineering wars in Palestine and Turkey. But they planned for it, which made it inevitable. Everything about castle design had to be different.