Shell keep

A shell keep is a form of medievalfortification, best described as a stone structure encircling 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 tower keep (though this was occasionally attempted, as at Duffus Castle, Moray). A common solution to this problem was to replace the palisade with a stone wall and then build wooden buildings backing onto the inside of the wall.