Shell keep

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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 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.

A gazetteer compiled by archaeologist Robert Higham counted 21 shell keeps in England and Wales.[1] Examples include the Round Tower at Windsor Castle[2][3] and the majority were built in the 11th and 12th centuries.[4]



  • Darvill, Timothy; Stamper, Paul; Timby, Jane (2002). England: an Oxford archaeological guide to sites from earliest times to AD 1600 (illustrated ed.). Oxford University Press. p. 196. ISBN 0-19-284101-7.
  • Higham, Robert (2016), Shell-keeps revisited: the bailey on the motte? (PDF), Castle Studies Group open access
  • Hislop, Malcolm (2013). How to read castles. London: Bloomsbury Academic. ISBN 9781472521613.
  • Pettifer, Adrian (2002). English Castles: A Guide by Counties (illustrated ed.). Boydell & Brewer. p. 7. ISBN 0-85115-782-3.