Shell keep

An aerial photograph of Windsor Castle, with three walled areas clearly visible, stretching left to right. Straight roads stretch away in the bottom right of the photograph, and a built-up urban area can be seen outside the castle on the left.
An aerial view of Windsor Castle with its shell keep (called "The Round Tower") prominent on its motte inside the middle ward (middle bailey)

A shell keep is a style of medieval fortification, best described as a stone structure circling the top of a motte.

In English castle morphology, shell keeps are perceived as the successors to motte-and-bailey castles, with the wooden fence around the top of the motte replaced by a stone wall. Castle engineers during the Norman period did not trust the motte to support the enormous weight of a stone keep. A common solution to this problem was to replace the palisade with a stone wall and then build wooden buildings backing onto the inside of the wall. This construction was lighter than a keep and prevented the walls from being undermined, meaning they could be thinner and lighter.