The Guard Tower is a tower in Age of Empires, the upgrade to the Sentry Tower available in the Iron Age. To research its upgrade technology at the Granary, 300 food and 100 stone is required. This defensive building has 200 hit points, 6 attack and 7 range and can be upgraded. It offers effective defense against other Bronze Age units, but needs to be upgraded to the Ballista Tower to have any kind of effectiveness against Iron Age units. Almost every civilization can build this type of tower. The only exceptions to this are the Romans, the Minoans, and the Yamato.
Like all towers, Guard Towers are more resistant against stone throwing siege weapons than other buildings, having 18 additional siege armor against them after accounting for building armor. Against Armored Elephants, Guard 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 Guard Tower was a superior fortification, well- designed for holding out against attack and for bringing weapons to bear on an attacker.
—Age of Empires manual
Guard Tower (left) and Ballista Tower (right). Notice that their appearance are the same; however, the former fires arrows, while the latter fires Ballista bolts.