Walls (Civ3)/Civilopedia

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.