Siege Attack

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

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. The number of units is 50 per acre taken, so an easy way to monitor if you have enough is to say 5000 units per 1000 acres of your opponent (Because 10% of the 1000 acres equals 100, and 5000/100 is 50.).
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 the Siege unit ability into account).
However, the accuracy loss is disregarded on defending units with FLYING.

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.