The Sentry Tower is a defensive structure available once the Bronze Age is reached. It is also available to the Scenario Editor and can be seen throughout the Campaign mode of the game. It is an upgrade of the Watch Tower and can be researched in the Granary for 120 food and 50 stone. Every civilization in the game have the ability to construct this Tower. Like all towers, Sentry Towers are more resistant against stone throwing siege weapons unlike other buildings, having 18 additional siege armor against them after accounting for building armor. Against Armored Elephants, Sentry Towers have 16 extra armor.
"The tower discovered on the wall at the ancient site of Jericho served several purposes. It extended the visual range of lookouts that would be watching for the approach of raiders and other visitors. An early warning might have been the difference between a successful defense and the fall of the town. The tower was a superior firing position for archery. Bowmen shooting down had an advantage in range and penetration power of arrows versus enemies shooting up. Enemies hiding at the bottom of the wall may have remained visible to archers in the tower. The tower itself was an independent bastion that could serve as the defensive position of last resort if the wall was carried. The Sentry Tower was an improved fortification of strong materials and designed for defense."