the Great Wall, were discovered along the border of Ningxia autonomous region and Gansu province. Characteristics Before the use of bricks, the Great Wall was mainly built from rammed earth, stones, and wood. During the Ming, however, bricks were heavily used in many areas of the wall, as were materials such as tiles, lime, and stone. The size and weight of the bricks made them easier to work with than earth and stone, so construction quickened. Additionally, bricks could bear more weight and endure better than rammed earth. Stone can hold under its own weight better than brick, but is more difficult to use. Consequently, stones cut into rectangular shapes were used for the foundation, inner and outer brims, and gateways of the wall. Battlements line the uppermost portion of the vast majority of the wall, with defensive gaps a little over tall, and about wide. From the parapets, guards could survey the surrounding land. Sticky rice mortar, consisting of sticky rice soup mixed with slaked lime, was extensively used to hold bricks together; no human bones or body parts were ever incorporated into the mortar or any part of the wall, contrary to what a legend states. Communication between the army units along the length of the Great Wall, including the ability to call reinforcements and warn garrisons of enemy movements, was of high importance. Signal towers were built upon hill tops or other high points along the wall for their visibility. Wooden gates could be used as a trap against those going through. Barracks, stables, and armories were built near the wall's inner surface. Condition While portions north of Beijing and near tourist centers have been preserved and even extensively renovated, in many other locations the wall is in disrepair. The wall sometimes provided a source of stones to build houses and roads. Sections of the wall are also prone to graffiti and vandalism, while inscribed bricks were pilfered and sold on the market for up to 50 renminbi. Parts have been destroyed to make way for construction or mining. A 2012 report by the National Cultural Heritage Administration states that 22% of the Ming Great Wall has disappeared, while of wall have vanished. More than of the wall in Gansu province may disappear in the next 20 years, due to erosion from sandstorms. In some places, the height of the wall has been reduced from more than