Forts are structures which exist on the map and can be upgraded. Forts serve as collection points for resources, supply depots for distribution to surrounding provinces, and shelter for troops in the event of a siege. Each fortress type has different attributes.
Admin The admin value of a fort determines the percentage of resources from neighboring provinces that the fortress can collect. It also propagates supply into nearby provinces. The formula for this is (Administration x 4) / (Distance + 1). Thus, a fortress with admin 50 contributes 100 supply to adjacent provinces. Four provinces is the maximum distance for this supply propagation.
Administration also increases the income of a province by Admin / 2%. Thus, a fort with an Admin value of 30 would increase the income by 15% of any province in which it is built.
Defense The defense value of a fort represents the number of points of damage that must be done to a fort by an enemy siege before it can be attacked. Each turn a comparison is made between the strength of the sieging and besieged forces at a fortress. The difference between these forces determines the amount of damage done to the fortress’ defense value.
Supply The supply value of a fortress determines only how many units can be supplied inside that fortress in the event of a siege. It does not affect the distribution of supply to surrounding provinces. Each turn a fortress is under siege, its supply value is divided by the length of the siege to determine the supply points available on that turn to the besieged units. Thus, on the fifth turn of a siege of a fortress with a supply value of 100, the fortress provides besieged units with 20 supply.
The statistics for each fort are listed when you click on the Fortification button in the province interface (also the [f] key). Some forts give bonuses to Commander Points and/or Recruitment Points.
Forts also have defense that functions as Province Defense when the fort is attacked. The current defense value of the fort against sieges is listed.