Note: The completion of 'An Oath is an Oath' also completes the 'Siege' quest, so it is a good idea to finish up any of the optional objectives in that quest line before you proceed.
When you have properly rested, eaten and reconditioned your attire, go to Sir Divish and speak with him about the plans for the siege.
With a successful hit on the walls, speak with Sir Robard.
You now have two days to do whatever you need to attend to, but do not forget to come back to the encampment when you are ready. If you have nothing else to do, simply use the time dial and wait for two days.
As the battle begins, run with the other men to storm the walls. If you are not confident in your combat ability, let a few soliders climb the ladders first to bear the brunt of the attack.
When you have secured the walls, fight your way to the outer bailey. There will be a lot of foes rushing in through the inner bailey, so it is a good idea to take the initiative and meet them at the gate.
The only condition of the agreement is that Sir Radzig rides with Istvan Toth until Skalitz to ensure that no one follows him. You must now ride with Lord Capon to collect Sir Radzig.