Siege Attack

The biggest form of attack in the game, involving a larger part of a kingdom, including its forts.

A succesful Siege attack gains land, including at least one fortress from the defeated mage.
The defender loses max 10% of his land (won Regular attack would give max 5%). The sieging attacker gains 1/3 of that, if the attacker was within 200% of defender net power before the battle (as in any attack).
To conquer the maximum amount of land in a siege attack, a sufficient number of units needs to have survived the battle to occupy each newly gained acre. The same goes to win forts.
For eradication mages ('s only!), the enchantment Battle Chant makes that you also capture geld (like in a Pillage) after a won Siege.

Siege attacks are more difficult to win than regular attacks, because:

Since forts are involved, the targeted mage is granted defensive bonuses when defending.
On top of that, the sieging army receives a 10% accuracy penalty (base accuracy is 30%, in a Siege it becomes 20%, not taking Marksmanship or Siege Unit Abilities in account).

A Siege takes 2 turns
A Siege can be blocked by Hallucination - the attacker loses 2 turns + war expense
The Battle Chant enchantment can give geld to a succesful invader of Eradication speciality.
Epidemics can be transferred from attacker to defender and from defender to attacker in Regulars.
Enchantments can be dispelled by Siege attacking, no matter if you win or not.