A Bunker is a small, cheap fortification, available in the Modern Age. It automatically attacks enemies within its line of sight. It can be garrisoned with friendly troops, which will increase its attack and will also slowly heal while inside the tower. The keep is preceded by the Stockade and does not have any more upgrades.
The bunker is the last upgrade in the tower upgrade tree. 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 and lone, scattered enemy units.
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.