style, often reusing Roman materials and sites, and were almost certainly intended to impress and generate a political effect amongst local people. The political value of these keep designs, and the social prestige they lent to their builders, may help explain why they continued to be built in England into the late 12th century, beyond the point when military theory would have suggested that alternative designs were adopted. The second early stone design, emerging from the 12th century onwards, was the shell keep, a donjon annulaire in French, which involved replacing the wooden keep on a motte, or the palisade on a ringwork, with a circular stone wall. Shell keeps were sometimes further protected by an additional low protective wall, called a chemise, around their base. Buildings could then be built around the inside of the shell, producing a small inner courtyard at the centre. The style was particularly popular in south-east England and across Normandy, although less so elsewhere. Restormel Castle is a classic example of this development, as is the later Launceston Castle; prominent Normandy and Low Country equivalents include Gisors and the Burcht van Leiden – these castles were amongst the most powerful fortifications of the period. Although the circular design held military advantages over one with square corners, as noted above these really mattered from only the end of the 12th century onwards; the major reason for adopting a shell keep design, in the 12th century at least, was the circular design of the original earthworks exploited to support the keep; indeed, some designs were less than circular in order to accommodate irregular mottes, such as that found at Windsor Castle. Mid-medieval keeps (late 12th–14th centuries) During the second half of the 12th century, a range of new keep designs began to appear across France and England, breaking the previous unity of the regional designs. The use of keeps in castles spread through Iberia, but some new castles never incorporated keeps in their designs. One traditional explanation for these developments emphasises the military utility of the new approaches, arguing, for example, that the curved surfaces of the new keeps helped to deflect attacks, or that they drew on lessons learnt during the Crusades from Islamic practices in the Levant. More recent historical analysis, however, has emphasised the political and social drivers that underlay these mid-medieval changes in keep design. Through most of the 12th century, France