Before you speak to Captain Robard, you will need to find some equipment which is suitable for a night raid. This requires darkly coloured, lightweight armour with minimal visibility and noise ratings. If you do not already have some, your best option is to buy it in Rattay or Sasau from the Armourer, Tailor and Cobbler.
Note: If you do not want to buy armour for the night raid, you can fight with bandits until you find enough suitable equipment by looting them.
If you can buy whatever you like, the most effective combination is the following:
-- Quiet dark shoes
-- Hunter's gloves
-- Dark Saxon gambeson
-- Short dark waffenrock
A suitable form of legwear can be more difficult to find. Your best option is to speak with the Miller's or find some black or brown hose from a fallen bandit.
Once you are confident that you have the proper equipment for the night raid, speak with Sir Robard in the camp by the Talmberg quarry.
As you approach the walls with the ladder, wait until the guards have gone up first to take out the sentries on the walls. When you are given the signal, climb the ladder. When you get to the top of the wall, listen to what the guard has to say and turn left to eliminate the sentries.
After waiting for the guard to move, follow Lord Capon to the sleeping quarters in the inner bailey.
After speaking with the villagers, consult with Lord Capon about what to do next. Regardless of what you decide to do, Lord Capon will be shot in the arse and you will have to carry him out.