As many mottes were man-made and liable to collapse under the weight of large quantities of masonry, sticking the traditional Norman-style rectangular tower on top of one was soon discovered to be Not A Good Idea. The alternative was the shell keep. This is in essence a curtain wall on a small scale, with some form of gateway and the usual range of domestic buildings, hall, kitchen, chapel, etc., built around its inside face. Restormel Castle in Cornwall is a nice example of the mode.

In some cases, the shell keep was constructed along with its motte, so that the walls of the keep penetrate the full depth of the motte and their foundations are at the natural ground level. This gives weight to the argument that motte and bailey castles were not exclusively conceived as temporary structures. (See "Motte and Bailey Castles")