Further developments were needed in order to make castles stronger. The next step in castle keep building came when the weak corners were taken away and keeps were made circular. In some cases a stone wall was built to replace the wooden palisade round the top of the motte in a round or polygonal shape. These keeps were known as shell keeps. A good example of a shell keep is the one at Cardiff Castle, built in the 12^th century.

Shell keeps were built from a few years after the Conquest to the mid-13^th century.