The list of buildings
Walls can be built only in town-size settlements and provide a land bombardment defense of eight, and a 50% defensive bonus to units defending in the town. When a town becomes a city, the walls no longer provide a defensive bonus.
Before the establishment of centralized governments capable of supporting strong, national armies, individual cities were left to fend for themselves when it came to defense. As a result, many cities constructed city walls to protect against raiders and bandits. City walls represented a major investment in both time and materials, requiring years to complete and constant repairs to maintain their strength and integrity. However, these walls turned the city into a fortress capable of withstanding all but the most determined attack.