<P> One factor was the introduction of stone into castle building . The earliest stone castles had emerged in the 10th century, with stone keeps being built on mottes along the Catalonia frontier and several, including Château de Langeais, in Angers . Although wood was a more powerful defensive material than was once thought, stone became increasingly popular for military and symbolic reasons . Some existing motte - and - bailey castles were converted to stone, with the keep and the gatehouse usually the first parts to be upgraded . Shell keeps were built on many mottes, circular stone shells running around the top of the motte, sometime protected by a further chemise, or low protective wall, around the base . By the 14th century, a number of motte and bailey castles had been converted into powerful stone fortresses . </P> <P> Newer castle designs placed less emphasis on mottes . Square Norman keeps built in stone became popular following the first such construction in Langeais in 994 . Several were built in England and Wales after the conquest; by 1216 there were around 100 in the country . These massive keeps could be either erected on top of settled, well established mottes, or could have mottes built around them--so - called "buried" keeps . The ability of mottes, especially newly built mottes, to support the heavier stone structures, was limited, and many needed to be built on fresh ground . Concentric castles, relying on several lines of baileys and defensive walls, made increasingly little use of keeps or mottes at all . </P> <P> Across Europe, motte - and - bailey construction came to an end . At the end of the 12th century the Welsh rulers began to build castles in stone, primarily in the principality of North Wales and usually along the higher peaks where mottes were unnecessary . In Flanders, decline came in the 13th century as feudal society changed . In the Netherlands, cheap brick started to be used in castles from the 13th century onwards in place of earthworks, and many mottes were levelled, to help develop the surrounding, low - lying fields; these "levelled mottes" are a particularly Dutch phenomenon . In Denmark, motte and baileys gave way in the 14th century to a castrum - curia model, where the castle was built with a fortified bailey and a fortified mound, somewhat smaller than the typical motte . By the 12th century, the castles in Western Germany began to thin in number, due to changes in land ownership, and various mottes were abandoned . In Germany and Denmark, motte - and - bailey castles also provided the model for the later wasserburg, or "water castle", a stronghold and bailey construction surrounded by water, and widely built in the late medieval period . </P> <P> In England, motte - and - bailey earthworks were put to various uses over later years; in some cases, mottes were turned into garden features in the 18th century, or reused as military defences during the Second World War . Today, almost no mottes of motte - and - bailey castles remain in regular use in Europe, with one of the few exceptions being that at Windsor Castle, converted for the storage of royal documents . The landscape of northern Europe remains scattered with their earthworks, and many form popular tourist attractions as part of the European heritage industry . </P>

Why is there very little evidence now of motte and bailey castles