storming castles

[Psitticine]

Basically, the strength of your forces are added up, bonuses for siegecraft (a special ability available to a few units, including the Ulmish Engineers) are factored in, and that is the amount you weaken the enemy's fort. The enemy, however, uses the same process with its strength and castle defense bonus (another special ability available to a few units) to rebuild the defenses.

You need to have not only a large force but one that sufficiently out-strengths the opposition. There are some spells that can help, high-strengh summons also help, and mindless units are to be avoided. (Mindless troops count only 1/10 of their strength towards sieging or defense.)

It can be time-consuming but at least that means your castles won't go down easily either.