First, decide where you want to assault the fort. The intended method is to attack the front gatehouse, although some may find it easier to assault the fort from the flank.
When you have your plan of attack, engage the perimeter defenses with your archers and crossbowmen. When safe, have a portion of you spearmen to fill the moat.
Break down the wall. It's best to target one of the perimeters or gatehouses. Use the battering ram, with support from tunnelers or macemen as required.
Once you break through, have your melee units finish off the remaining forces.