Obsolete with Rifling (Except defensive bonus) (only in and )
"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. The development of gunpowder largely put an end to the usefulness of walls in stopping large armies."
"Walls increase a city's defenses against pre-gunpowder units. Build them in cities under threat from enemy or barbarian assault."
In the city screen press the following icon to start Walls construction: