A shell keep is an enclosure, of masonry, extending round the top of an earlier motte or castle ringwork and replacing the existing timber palisades; there are a few cases where the wall is built lower down the slope or even at the bottom (as at Farnham, Surrey), when it acts as a revetment. The enclosure is usually round or sub-rounded but other shapes are known. The wall will be easily recognisable when substantial parts remain, especially if the wall is full height, when it may still be crenellated (as at Restormel, Cornwall). It may be seen as slumped masonry (Lewes Castle Brack Mount, Sussex), or only found by excavation (Aldford, Cheshire). Shell keeps should be evaluated as separate, but superimposed, monuments from that which went before them. The main components are the shell wall itself, and other major components may be gateways, mural towers, a central tower, and steps. Shell keeps, as with mottes and ringworks, are often associated with one or more baileys.

Although shell keeps are usually easy to identify if on a mound, they may, if on a large ringwork, be confused with another monument class, the enclosure castle. A shell keep will be relatively small, probably between 15m and 25m diameter, usually circular or sub-rounded, with few buildings, and perhaps one tower only, within its interior; an enclosure castle will be larger, often of irregular shape more than 50m in width and 100m in length, possibly with several mural towers, some large internal buildings, and perhaps a tower keep.

Shell keeps could be defended habitations, or military strongholds built for offensive operations, and are found in urban and rural areas. They were apparently built on existing mottes or ringworks, being effectively a replacement in masonry of the earlier timber structures, and many developed into major stone castles of various types. Shell keeps were built from a few years after the Conquest to the mid 13th century.