Keep

The Keep is the upgrade of the Guard Tower, and is the final highest level of arrow-firing tower you can get (The Bombard Tower is a further tower you can get upon upgrading to Keep, however it is unrelated and you can build the two types of tower simultaneously). Upgrading to Keep is expensive, and rarely pays off, as it is only a negligable improvement on the Guard Tower, however Keep is needed as a prerequirement to researching Bombard Tower, which is a more powerful but slower-firing gunpowder cannon-firing tower.

Keeps are strong against infantry and cavalry, particularly after Murder Holes, Masonry, Architecture and most other University technologies have been researched.