Quote Originally Posted by Tomcat
To those who suggested acquiring siege equipment by any means possible, I should just add that I wanted to know the best way to assault without using any siege units on this occasion.
Tomcat
One word: Autoresolve. This is effective at any time in the game, whether you are attacking a simple fort or a massive fortress.

I almost never build siege equipment (at least, until cannons become available). When I take a province, if a large number of enemy troops retreat to the castle (resulting in a short siege), I wait it out. If a small number of troops retreat to the castle (resulting in a long siege), I assault and autoresolve; my shear weight of numbers usually results in a decisive victory with few men lost.

You can try various techniques for more effective manual assaults, but you will rarely get as good results as you get with autoresolve.