You can easily take a wooden fort without seige equipment. Just order a group of soldiers to attack a wall. The fort only has one small arrow tower. You probably won't loose 10 men knocking down the wall. After that, it's just a matter of attacking the soldiers inside with their counter unit.

Also, If you have the VI expansion, you can tell an archer unit to fire at the walls. They will automatically use fire arrows, which will destroy wooden walls very quickly. They won't fire at stone walls, of course.

As far as a keep is concerned, just use some cheap units like urban militia to attack the front gate. If you are playing VI, the enemy will take this opportunity to pour boiling oil on them. This causes a lot of casualties. So, order them to attack the gate, and as soon as they make contact, order them to back off a few paces. The oil will miss you every time. The castle only has 5 or 6 shots of oil. Once they are gone, you are free to wreck the gate. Just keep the good units of your army back and well out of range of the castle's defenses. You will take a lot of casualties, but they will be cheap militia-type casualties.