Shell keep

An aerial view of the Windsor castle: with its shell keep (called "The Round Tower") prominent on its motte inside the middle ward (middle baily).

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 was to replace the palisade with a stone wall 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.

Examples include the Round Tower at Windsor Castle and Clifford's Tower at York Castle, and the majority were built in the 11th and 12th centuries.

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  The shell keep of Windsor Castle was built by Henry II and remodelled in the 19th century.

• York castle exterior.jpg

  York Castle's Clifford Tower: A shell keep on a motte

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  A cross-section of York Castle's shell keep and motte, produced in 1903 by Sir Basil Mott; "A" marks the 20th century concrete underpinnings of the motte; the low walls enclosing the base of the motte are a 19th-century addition.

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  Restormel Castle built around the motte

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  Plan of Restormel Castle