Sieges disable recruitment of units in the province. The besieged nation cannot move units into or out of the fort unless they are stealthy.

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).

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.

Iron Walls can increase a forts maximum wall integrity. This also increases the current wall integrity by the same amount. If forts wall integrity is already at 0 a Iron Walls cast on it can not stop fortress storming occuring 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 to late.)

The besieger each turn get's a message with a report on the progress.

"Work is going very slowly" means less than 15% wall integrity damage was dealt last turn.

Calculations requested?

Siege Strength = Unit's Strength^2 (-_ if mindless -_ if animal -_ if undisciplined)/ 100

Siege Defense = Unit's Strength^2 (-_ if mindless -_ if animal -_ if undisciplined) / 200

Sallying troops and allied units attempting to relief 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).

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.