|Upgraded From||Guard Tower|
Upgrading to Keep is expensive, and rarely pays off as it is only a negligable improvement on the Guard Tower. They are only avaliable to Britons, Byzantines, Celts, Chinese, Japanese, Koreans, Maya, Saracens, Spanish, Teutons and Turks.
Garrisoned foot archers and villagers will increase the number of arrows the tower fires, thus increasing the damage. Up to five foot-soldiers (infantry and archers, although infantry do not increase the firepower of the tower) may be garrisoned into the watch tower. When garrisoned in the keep (or any other building) units will slowly regain any lost hit points. This ability can be sped up by researching Herbal Medicine.
Offensively, towers can be upgraded by Fletching, Bodkin Arrow, and Bracer (all Blacksmith upgrades) for range and damage, Chemistry (at the University) for damage, and Heated Shot (also at the University) for damage against ships. Defensively, towers may be upgraded with Masonry, Architecture (increase HP), and Murder Holes (eliminates minimum range). Towers are generally more useful in the game for stopping and slowing small raiding parties. They are ineffective against large armies and siege weapons.
The central and strongest building inside a castle was called the keep. This was the last bastion of defense inside if the walls and outside towers were taken. The remaining defenders withdrew into the keep for the final defense. The keep was a mini-castle often constructed in place of a castle to cut expenses. Although much smaller than castles, keeps could perform the same function on a smaller scale. They were very difficult to capture except by a large and well-equipped army.