[MaxWilson]

It takes a long time. Examine one of your own fortifications (click on the "Fort" icon off to the right in a province where you have a fort, like your capital) and you'll see it has a defense value of between 200 and 1200, depending on what type of fortification it is. Roughly speaking, every attacking unit reduces that defense by 1 per turn and every unit in the castle repairs 1 per turn, so if you want to take down even a wimpy fort quickly you'll need a large army.

You have the option of forging a couple of items that can make sieging quicker (Construction-4 gives the Horn of Valor, Construction-6 gives the Gate Cleaver, which are worth +25 and +50 per turn respectively), and some nations (Arcoscephale, Agartha) have units that receive a bonus to sieging, but pretty much you need to have a big, cheap army and be patient.

-Max