Forts are the symbol of an archmage's power, as well as the defensive bastions of his military. When a kingdom's last fortress is destroyed, its ruler is slain and will need to reincarnate a new soul from Hell to return to Terra.
Each fort adds a value of 19,360 to its kingdom's net power. However, forts beyond the first 2.5% of a kingdom's land do not (since May 2008) add further net power.
Each fort also provides residential space for 500 people.
Health Points Bonus
Forts above a certain level also give a bonus to the health points of all units in a defending army. A base HP bonus of 10% on regular attacks and 20% on siege attacks always applies. Having more than 0.67% of kingdom land in forts makes the bonus increase linearly until a maximum bonus of 37.5% on regulars, and 75% on sieges, is reached at 2.5% of kingdom land.
The upkeep of forts differs from that of other buildings in that each additional fort built has a higher upkeep than the last one. This means that building a large amount of forts can very quickly cause a significant increase in geld upkeep.
The upkeep Un of the nth fort is:
Un = 240 + 60n
It follows from this that the total upkeep U(n) for n forts is:
U(n) = 240n + 30n(n+1)
As a rule of thumb, keeping 1% forts is usually a good idea. It provides a useful defensive bonus and security in the event of hostility, without costing large amounts of geld to upkeep, or raising the kingdom net power so much that mages with a much bigger army will be in range to attack.
Following changes to oversummoning, Fort Halling was a decent way to gain power and rank during Armageddon. This is no longer the case because the Forts above 2.5% do not increase your power design change.