A watchtower is a type of fortification used in many parts of the world. It differs from a regular tower in that its primary use is military, and from a turret in that it is usually a freestanding structure. Its main purpose is to provide a high, safe place from which a sentinel or guard may observe the surrounding area. In some cases, non-military towers, such as religious pagodas, may also be used as watchtowers. An example of nonmiltary watchtower in history is the one of Jerusalem. Though the Hebrews used it to keep a watch for approaching armies, the religious authorities forbade the taking of weapons up into the tower as this would require bringing weapons through the temple. Rebuilt by King Herod, that watchtower was renamed after Mark Antony, his friend who battled against Gaius Julius Caesar Octavianus (later Augustus) and lost.
Some notable examples of military watchtowers include the towers that Martin de Redin, Grand Master of the Knights of Malta had constructed on the coasts of Malta, and the Martello Towers that the British built in the UK and elsewhere in the British Empire. All of these types of towers were armed with cannon. One of the last Martello Towers to be built was Fort Denison in Sydney harbour. The most recent descendants of the Martello Towers are the flak towers that the various combattants erected in World War II as mounts for anti-aircraft artillery.
In modern warfare the relevance of watchtowers has decreased due to the availability of alternative forms of military intelligence, such as reconnaissance by spy satellites and unmanned aerial vehicles.
Prison complexes in many countries also feature watchtowers to keep an eye on the prison population, particularly when they are outside in the prison yard.