A shell keep was a stone walled enclosure built around the top of a motte. A shell keep is relatively small, probably between 15m and 25m diameter, usually circular with few buildings, and perhaps one tower only, within its interior. 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.