How did the motte and bailey castle change to the shell keep?

The motte and bailey castle evolved into the shell keep as a response to the need for stronger fortifications and improved defenses. The original motte and bailey design, characterized by a raised earth mound (motte) and a courtyard (bailey), was often vulnerable to siege tactics. Over time, stone walls were added to create a shell keep, a circular or polygonal structure that provided a more durable and secure stronghold. This transition reflected advancements in military architecture and the increasing importance of permanent, fortified structures in medieval society.