Why were castles developed?

Castles changed because the design improved. Though the later ones took longer and were more expensive to build, they were much better as forts. The first castles were of the motte and bailey type, built of earthworks and wood. They did not last very long, and important parts of the defenses could be overpowered easily or burned down.

Later castles were more strongly built, usually of stone, and had more elaborate defenses. They could withstand both time and assault better.