Shell keep

An aerial view of the Windsor castle: with the 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. They are very good at defending from an attacker.

While some authorities describe the Round Tower at Windsor Castle and Clifford's Tower at York Castle as surviving examples of shell keeps, some others state that to be a shell keep the walls must surround the bottom of the motte as with Berkeley, Farnham and Restormel castles.

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

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  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

References

• Brown, Reginald Allen (1989). Castles from the air:Cambridge air surveys (illustrated ed.). CUP Archive. p. 52. ISBN 0521329329. 
• Timby, Jane (2002). England: an Oxford archaeological guide to sites from earliest times to AD 1600 (illustrated ed.). Oxford University Press. p. 196. ISBN 0192841017. 
• Hull, Lise (2006). Britain's medieval castles (illustrated ed.). Greenwood Publishing Group. p. 99. ISBN 027598414. 
• Pettifer, Adrian (2002). English Castles: A Guide by Counties (illustrated ed.). Boydell & Brewer. p. 7. ISBN 0851157823.