Royal Knights proved effective in bringind down seige engines and getting back int the castle. Sometimes they can get attacked coming back through the gate, jamming it open and causing a disaster. In this case it is better to run the unit on a fast patrol around the castle walls keeping the attacking unit under fire and away from the gate.

Will organ guns not damage my own gate? Or do I have to wait until they are opened before firing? I will have to try them out again using the staggered tactic.
The most important lesson I learned is that playing out sieges is fun and worthwhile as well. The attacker will always send at least one elite unit to break down gates and they will take most of the defenders fire so it pays to not resolve them automatically. It's also a great way to get rid of the enemy's heir units as the AI thinks nothing of throwing them against your castle walls to their death. So even if you have just one man in that castle play the battle out, your defeat is probably inevitable but the cost to the enemy will be plain to see as you survey the corpses piled around your castle.