Stone keeps replaced Motte and Bailey castles because the keeps had many defensive mechanisms unlike the Motte and Bailey that were open to being set on fire. In addition, Stone castles were more durable unlike the Motte and Bailey that were made of wood.
Because they were stronger and more efficient
Motte and Bailey castles were replaced by stone castles due to their greater durability, improved defensive features, and the evolving nature of warfare. Stone castles could withstand attacks better and served as more effective symbols of power in the feudal system. This shift reflected advancements in military technology and the need for stronger fortifications.
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