Although Normans began to build castles in order to hold onto their English land–and then later to fight off their French cousins from seizing their Norman land–castles didn’t really come into their own until the Normans became the backbone of the First Crusade. Although people from all over Europe joined the Crusade, the Normans–both from England and in the old Frankish lands–were a disproportionately large fraction. It may be due to inheritance reforms that prevented landowners from splitting their estates up or giving too much away to the church. It was better for the king if the oldest son inherited a huge estate to support the military-training and horsebreeding structure. But this left a lot of younger sons who had spent their lives training in war and had nothing to inherit.
The Holy Land had been a battlefield for centuries already. The Byzantine Empire had fortified the towns against Persian attack, only to have them fall to the much smaller Arab armies in the wake of a plague. Arabs had copied Byzantine structures to improve on fortifications, and additionally some of the most ancient fortified walls in the world were in this region. So once the castle-builders were able to seize cities, they could study foreign building methods.
Next, in order to hold their new cities, they built their own forts and castles all over Palestine, Syria and Turkey. They wanted to innovate as well as copy, in order to find new things that the enemy would not know how to overcome. The First Crusade was able to seize a lot of trading wealth in the region, so at first money was no barrier.
Crusader castles were built all of stone (instead of the stone and lumber methods of Northern Europe), partly because there was so little timber in the dry Near East. They designed around the concept of concentric rings with towers to look out over the land, instead of the early Norman “motte” that often sat on an artificial hill. The concept of defense had not changed: keep the besiegers at bay long enough for outside help to arrive. But the ways to delay their attackers became many and varied. (next: a typical Crusader castle)