Well defending seems to be very easy for me, whether or not I have my own missile troops or even a wooden wall its all very simple. I held Antwerp with 5 units of town militia spearmen, against a Danish stack of dismounted huskarls and Warrior priests (or whatever). Similarly I held Rennes with an army of peasants and militia against a much larger and more experienced French army.

The trick is to pack the gate area with spears, keep any cavalry in reserve till they are needed, and don't focus too much effort on holding the walls - they are useful while the enemy is advancing but once they have broken in then all their efforts are on fighting their way to the main square. If they take the walls then they still have to do all of that.