Citadel (Civ5)/Civilopedia

A Citadel is a mighty fortification that can only be constructed by a Great General, usually near a city or on a particularly defensible piece of terrain. The Acropolis in Athens, Greece, is an example of an early powerful Citadel. Such structures were almost impossible to take by direct attack, and they were capable of withstanding protracted sieges before they fell. You can construct a Citadel anywhere within your territory, or directly adjacent to your territory. Upon constructing the Citadel, your Culture borders will also expand to surround the Citadel on all sides by one hex. If the new Culture border claims hexes already owned by another civilization, you will incur a diplomatic penalty as a result. Any unit stationed within a Citadel receives a 100% defensive strength bonus. Additionally, any enemy unit which ends its turn next to a Citadel takes 30 damage (damage does not stack with other Citadels).