Walls are replaced by the following unique building:
Walls increase a city's defenses against pre-gunpowder units. Build them in cities under threat from enemy or barbarian assault.
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.