Improvement

Terrains can be modified by human-made (worker units) features called improvements. These improvements are summarized in the following table:

Improvement	Requirements	Bonus yield	Economic bonus	Military bonus	Upgrades to
Fortified cave	Shelter Building			+15% tile defense. Acts as a city for combat purpose. +1 Visibility height. +50% Air defense.	Fortified cave with cache
Fortified cave with cache	Pottery			+25% tile defense. Acts as a city for combat purpose. +1 Visibility height. +90% Air defense. Influences tiles in range 1.	Fort
Fort	Mathematics		Provides any tradeable resource in the tile.	+50% tile defense. Acts as a city for combat purpose. +1 Visibility height. +2 Visibility range. +20% Air defense. Influences tiles in range 1. Provides ZoC.	Command bunker
Command bunker	Electricity		Provides any tradeable resource in the tile.	+75% tile defense. Acts as a city for combat purpose. +1 Visibility height. +2 Visibility range. +40% Air defense. Influences tiles in range 1. Provides ZoC.	Command center
Command center	Metamaterials		Provides any tradeable resource in the tile.	+100% tile defense. Acts as a city for combat purpose. +1 Visibility height. +2 Visibility range. +60% Air defense. Influences tiles in range 1. Provides ZoC.