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.
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- 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.
- Hull, Lise (2006). Britain's medieval castles (illustrated ed.). Greenwood Publishing Group. p. 99. ISBN 0-275-98414-1.
- Pettifer, Adrian (2002). English Castles: A Guide by Counties (illustrated ed.). Boydell & Brewer. p. 7. ISBN 0-85115-782-3.