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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- Hislop, Malcolm (2013). How to read castles. London: Bloomsbury Academic. ISBN 9781472521613.
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