Motte and bailey castles had serious disadvantages in a siege. Timber
walls and towers were especially vulnerable to fire, and a castle that could
be set on fire with torches or scorching arrows provided only a short-term
solution to the need for defense. Obviously, walls had to be converted to
stone as soon as possible, but years had to pass before an artificial mound
had settled enough to bear heavy stone masonry. Where a natural hill
was available, especially a cliff beside a river that formed a natural water
barrier, stone towers and walls were being built by the end of the eleventh
century.
World History
