Definitions for "Shell keep"
Castle consisting of a high strong ring wall with residential buildings on its inner face. Examples: Clifford's Tower - York, Arundel Castle
The old motte-and-bailey castles were generally wooden stockades. As power was consolidated, the richer Norman lords built round stone walls on top of their mottes which were thus rendered fireproof. (At the same time, the Bailey curtain wall was also built up in stone.)
a stone wall on a motte