Defending archers do not do enough damage in siege.

[Didz]

I usually build two seuge towers and two ladders. I find the ladders actually more effective than the seige towers in scaling the walls but I usually send the towers in first becuase they are slow and then run the ladders to the walls after they have dropped their ramps. So I get to choose points where the wall is poorly defended for my ladder assault, and usually run my crossbowmen onto the walls this way.

Once On the walls I capture the nearest gate to let the rest of the army in and then use the walls to gain access to the inner fortess.