I did not read anything in the "how to build a wall" section about moats. Several upgrades were mentioned about fortifying walls, but this was not...the only game I ever played with a moat was Lords of the Realm II. I don't know if adding this would be unlikely due to the impenetrable castle syndrome that you are trying to avoid, but I would like to see a moat upgrade.
A second thought is that also I remember reading that armies often sent tunnelers underneath walls to weaken them and induce a collapsing. And the besieged enemy sent counter tunnelers. I have NEVER seen this in a game. Could this perhaps be the poor man's way into the castle?