Forests are tile improvements that are present in temperate or cool areas of the map. They give 25% increased strength to defenders and have a movement cost of 2.
Cities built near forests recieve a 0.25 health bonus per forest
The only improvements that can be built on Forest tiles are Roads and Lumbermills. This does not apply to workers of the Elven civilizations, who can build any improvement that would be possible without a forest on the tile.