There was a large degree of variation in the size and exact shape of the castles built in England and Wales after the invasion. One popular form was the motte and bailey, in which earth would be piled up into a mound (called a "motte") to support a wooden tower, and a wider enclosed area built alongside it (called a bailey); Stafford Castle is a typical example of a post-invasion motte castle. Another widespread design was the ring work in which earth would be built up in a circular or oval shape and topped with a wooden rampart; Folkestone Castle is a good example of a Norman ring work, in this case built on top of a hill although most post-invasion castles were usually sited on lower ground. Around 80 per cent of Norman castles in this period followed the motte-and-bailey pattern, but ring works were particularly popular in certain areas, such as south-west England and south Wales. One theory put forward to explain this variation is that ringworks were easier to build in these shallow-soil areas than the larger mottes.