

Starting bias - Many civilizations tend to favor a particular type of terrain for their homeland.Unique buildings are effective throughout the rest of the game, whereas unique improvements, like the regular ones, will provide nothing if pillaged. Both have additional special features which give the civilization some sort of gameplay advantage. While each unique building replaces a generic one (as with unique units), all unique improvements are in addition to normal ones. This is possible if the given civilization only has one unique unit.

Unique building or improvement - Many civilizations also have a unique building, and some even have a unique improvement instead.This means that the civilization will retain its unique units' advantages in subsequent eras, as long as they keep upgrading the same units instead of producing new ones. But in many cases, their unique abilities may transfer to more modern units when they're upgraded. Unique units are generally most effective during the era they appear, and often possess unique features and/or abilities that the regular ones do not. Each of these units replaces a generic unit of a given era. Unique unit(s) - Each civilization has at least one unique unit, and there are also some civilizations with a second unique unit.

The unique ability is always active throughout the entire game, although its efficiency might not be the same as the game progresses.
