Sieges disable recruitment of units in the province. The besieged nation cannot move units into or out of the fort unless they are stealthy. The besieger treats the province as if it is friendly.
During a siege income of the province is split between sieger and besieged.
Gems from sites all go to the besieged (a lab is required in the fortress to actually get them in the national treasury).
Priests of the sieging force can preach; if they do so their siege strength is then not counted for sieging.
The wall integrity of the fortress is reduced each turn by the difference between the sieger's siege strength and the defender's siege defense. If the defender's siege defense is higher the wall integrity increases (up to the normal maximum amount).
In principle, the supply available to the defenders is reduced each successive turn that the siege lasts. This effect is only very rarely relevant.
Forts have some intrinsic defenders based on the nation and the level of fort. These will automatically contribute some amount of siege defense.
When the wall integrity is 0 the command "Storm fortress" can be given to assault the fort. A quirk of the rules is that _any_ commander, whether or not it belongs to the sieging nation, can storm the fort, but will have to fight the besiegers first. This means that a stealthed army can be instructed to storm a fort that another nation has cracked; they will fight the besiegers and then whatever is inside the fort.
Iron Walls can increase a fort's maximum wall integrity. This also increases the current wall integrity by the same amount. If the fort's wall integrity is already at 0 an Iron Walls cast on it can not stop fortress storming occurring on that very same turn. (Wall integrity only prevents the issuing of the command, not the actual storming, so if the command is already given it's too late.)
The besieger each turn gets a message with a report on the progress.
|Message text||% of remaining wall integrity|
|Lightly||99 - ?|
|Moderately||85 - 51|
|Severely||50 - 11|
|Critically||10 - 1|
"Work is going very slowly" means less than 15% wall integrity damage was dealt last turn.
The following equations are rounded to the nearest tenth (0.1).
Siege Strength = (Unit's Strength^2 * 2 if Flying/ 100) + Siege Strength bonus
Siege Defense = (Unit's Strength^2 * 2 if Flying/ (100 * 8 if mindless * 2 if animal1) * 2 if undisciplined) + Castle Defence Bonus)/2
Sallying troops and allied units attempting to relieve the siege from another province fight in the same battle.
All magic phase movements of the besieged nation end inside the fort, therefore magic phase attacks on a sieging army are impossible (for the besieged player, does not apply to neutral remote attacks ala Send Horror). If you want to gem-burn a besieging army, negotiate with a third party!
Mages retreating from a sally will with very high probability retreat back inside the fort. However they decide the province they retreat to after all battles are fought, so if the fort is stormed and conquered on the same turn they will retreat to adjacent provinces like from a normal field battle.
Besieged Commanders that retreat die.
Ethereal units can ignore walls.
Intrinsic defenders are missile units (usually) that will appear on the fort walls as it is being stormed and fire on attackers. The walls provide them significant protection from missile fire coming from outside the fort.2)