Ancient cities were often built with special fortifications to protect the inhabitants from attacks from the outside. Sometimes the fortifications were natural, for example, on the top of a steep hill or next to a river. Man-made fortifications included at least a high, thick wall, usually made of bricks or stones. A city wall had one or more entrances, which were closed with gates. The gates were usually made of wood. The entranceway itself was often quite complex, involving one or more turns and several guardrooms. The gate, as well as the rest of the wall, could be supplemented with towers that stood up higher than the wall. These towers served both as vantage points from which to see someone approaching from a distance and also as high points from which missiles could be dropped or thrown on an attacking enemy.