The Citadel is the centerpiece structure of your city. Much of what you can do is determined by the level of your Citadel. Upgrading the Citadel unlocks additional building slots, construct and upgrade buildings, statues, and ultimately access various parts of the game.
Starting at level 10, your Citadel will receive a durability rating. Losing defensive battles in your city — on the Wall — will set your Citadel on fire, dealing damage. Fires can be put out ahead of time, and the Citadel has to be repaired afterwards. If your city burns down, you will receive hefty penalties to your construction and research, as well as in other areas.
You can find various useful information on your city in your Citadel, such as city resources, structures, boosts and more.
Requirements: Wall, House, Barracks, and an extra building (varies by level), Aurum