"Great Generals" are the supreme warriors of civilization. They create the weapons and train the soldiers. They plan the strategies to protect a civilization from its enemies, creating bold offensives, strong fortifications and cunning traps. They are the first out of the trench or onto the enemy's ship. They lead the charge into battle, carrying the fate of the people on their broad shoulders.
The Great General can construct the special Citadel improvement, which gives a big defensive bonus to any unit occupying it and damages enemy units that move adjacent to it. The Great General is consumed when it builds the Citadel. In Gods & Kings and Brave New World, this ability also has similar properties to the "Cultural Bomb" ability, adding all adjacent tiles into your territory. Be warned that stealing the territory of other civilizations can cause diplomatic incidents with the leaders.
The Great General provides a 15% combat bonus to all player-owned land units within 2 tiles. The Great General is NOT consumed when he provides this bonus. It should be noted that this buff to other units does not stack if you have multiple Great Generals.
The use of Great Generals, as well as Great Admirals, is much different than that of other Great People. They usually lay dormant until a war requires their abilities, at which point they should go immediately to the battlefield or sea. If you're fighting a lot, you may find yourself with more Generals/Admirals than you need - that's the time to construct Citadels in strategic locations (either to strengthen a weak point of the battlefield, or to conquer a tile with an important resource). Strictly speaking, unless you're pursuing a total war strategy on several battlefronts, you rarely need more than 1 Great General and Great Admiral.
When Garrisoned in a city, opens an additional Trade Route slot and provides newly-trained Infantry units in that city with the Trial Of The Arrow promotion, granting a random and unique ability after their first round of combat
Cannot build a Citadel but acts as one when in friendly lands
can be expended in the land of a City State to conduct a Diplomatic Mission that does not yield Gold but a free military unit immediately and every 25 turns afterwards (standard speed) as long as that City State is friendly
Cannot build Citadels, found Colonies, or join the Advisory Council.
Receive a free Carrocio when a Civilization following your religion and with whom you are at peace spawns a Great General. That Civilization also receives a free Carrocio, but will lose it if they declare war on you.
Units beginning stacked or adjacent to this unit heals 5 damage, yield 5 Faith, and lose any city attacking penalties. 10 damage is inflicted upon this unit each time that it heals other units.