A Keep is a small, cheap fortification, available in the Medieval Age. It automatically attacks enemies within its line of sight. It can be garrisoned with friendly troops, which will increase its attack and those units will also slowly heal while inside the tower. The keep is preceded by the Tower and followed by the Stockade.
Keeps upgrade to Stockades and Bunkers in the Enlightenment and Modern Ages respectively, given a high enough level of military technology. There are two types of research available at the tower. The first is the line of attrition upgrades, which cause increasingly large amounts of damage on enemy units within the player's territory without Supply Wagons. The second is researching and upgrading of militiamen, allowing Citizens to convert into Militia, then Minutemen, then Partisans as well as granting them more health. Although they will not stand up to any regular contemporary army, they can be used to hopefully harass enemy supply wagons.
Towers are not nearly as sturdy as Forts but are significantly cheaper; this makes them more of a stopgap measure. Determined enemy raiding parties can destroy lone towers even without siege equipment. All siege equipment have a greater range than towers.