Great tile improvement. Constructed by a Great General on or adjacent to your territory.
- Connects any strategic resource on the tile.
- +100% Defensive Combat Strength for (a friendly) unit on the tile – will not assist invading units.
- Damages all enemy units that end their turn on an adjacent tile for 30% of their maximum HP. (Damage does not stack with other Citadels.)
- Instantly annexes all nearby tiles into your empire's territory. (Must be constructed on a tile within or bordering your own territory. Will NOT annex an enemy city if placed adjacent to it, but will damage all units within it, including aircraft.)
Citadels have a variety of strategic uses, both during war and times of peace. Place Citadels at choke points in terrain (e.g. rough terrain like hill ranges) to defend your borders, forcing would-be attackers to either suffer heavy losses or attack from a less direct path. While they do not exert a Zone of Control to block fast units, they automatically "attack" by inflicting 30 percentage points of damage (i.e. 30% of max health) to any enemies stationed nearby.
Unlike the capturable Fort, you cannot place your own Citadel over top of an existing Citadel in foreign territory, even if it's adjacent to your own borders, nor can you build it outside your borders.
Citadels in wartime will continue to cause automatic damage to enemy units, including enemy units inside the Citadel or an adjacent city. They are still susceptible to enemy pillaging; while they will cease to function, unlike a regular Fort improvement they may be repaired.
In the expansions, even while outside of war, Citadels can be used to immediately acquire neighboring land tiles that contain a variety of important resources (much like the Great Artist special action). This essentially allows you to steal strategic or luxury resources from either an enemy civilization or a nearby city-state. Stealing land from enemy civilizations causes a diplomacy penalty, so stealing from a more powerful opponent isn't advisable. On the other hand, stealing from city-states only incurs an Influence decrease and doesn't seem to anger their allies.
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).