The Fortress is the heart of your City. Rule your citizens wisely. Monitor your Resource Production and adjust the Tax Rate by clicking on it. The game starts with a level 1 Fortress already built.
The look of your Fortress depends on the Race of character you choose at the beginning of the game.
The level of your Fortress determines how many fields you can build as well as how many wildernesses you can own. This information is shown by clicking on the Fortress, see the picture below for more details.
Note: The power boost received from the Fortress is the same as you would receive from the Wall or the Dragon Keep. The power you receive is cumulative, therefore for a lvl 10 Fortress you receive the power from lvl 1 + lvl 2 + lvl 3 and so on until lvl 10. The total power that you can receive is therefore 25,600.