A Watchtower or watch tower is a very high tower. They were used at city walls to see invaders, when they were far away so the city could prepare for an attack. A watchtower is also used for watching over people (like a town or a city). Watchtowers were popular in the Middle Ages. They were often used to watch over prisoners and make sure that they don't escape or leave the area, like in the Japanese Internment Camps after the December 1941 attack on Pearl Harbor.