They were very vulnerable to attacks using fire and the wood would eventually start to rot. Due to these disadvantages, King William ordered that castles should be built in stone. Many of the original timber castles were replaced with stone castles.

Stone keep castles were built primarily to provide a permanent, fireproof, and near-impregnable defensive stronghold for medieval nobility. Early "Motte and Bailey" castles were made of wood, which was quick to build but vulnerable to fire and rot. By transitioning to stone in the 11th and 12th centuries, lords could create massive "keeps" (the central fortified tower) that served as a final line of defense during a siege. These stone walls were several meters thick, making them resistant to battering rams and early siege engines. Beyond military defense, stone keeps were symbols of power and permanence; they broadcasted the lord's wealth and authority to the local population and rival landowners. They also served practical roles as administrative centers, granaries, and residences that could withstand long-term occupation. The shift to stone represented a move from temporary conquest fortifications to permanent regional control, ensuring that even if the outer walls were breached, the occupants could survive in the keep until reinforcements arrived.