Defensive wall

Defensive walls were common from the ancient period into the medieval period. Generally, these are referred to as city walls, although there were also walls, such as the Great Wall, which extended far beyond the borders of a city and were used to enclose vast regions.

In the 1980s Morocco built a system of sand wall defenses to keep back the Polisario.

The German Democratic Republic claimed that the Berlin Wall was defensive, but it was rather to prevent unauthorized emigration.

City walls were still occasionally used as late as the 19th Century although by this time they were generally of wood (rather than stone) construction and used only around small frontier settlements. City walls also had towers and were frequently surrounded by trenches.

The practice of building these massive walls had been developed sometime before the rise of the Sumerian Empire and was connected with the rise of city-states.

Oftentimes the walls proved impenetrable to attacking armies which then laid siege to the city.

Within walled cities, the poor and "noxious trades" were generally located near or outside the walls.

Chinese cities occasionally have remnants of city walls that were built in the Ming Dynasty and designed to withstand artillery bombardment. Chinese cities generally outgrew their walls, which fell into disrepair in the Qing dynasty. One example is Xian.

Portions of a Roman wall are still standing in Barcelona, and many Spanish cities, such as Avila[?] and Toledo, have medieval walls.

The remnants of the city walls of York are both a shortcut above the streets and, as in many places, a tourist attraction.