This is the first mission of chapter 8 in the Path of War, in Stronghold 2.
After successfully completing the defense of the abbey and seeing the cutscene at the end of it, you receive a message from a desperate Sir Grey. When you take over his castle, set up your economy and get weapons production going ASAP. It is useful to place 6-7 apple farms, 2-3 more woodcutter posts, another guard post and some stocks. Don't bother starting stone production yet because if you look right next door, you find a yellow castle belonging to Duke de Montparnasse. However, in this mission, just ignore him because he can't attack you yet.
Because you have very little time to save Sir William, It is best to set up a siege camp and a mercenary post to get out the ballistae and extra troops to take out the troops of Sir Hugo. Just keep out of range of his archers and ballistae. So long as you do so, it should be easy to take out the first section of troops. Move carefully, but quickly along towards the castle, destroying ballistae and trebuchets as you go. Take out as many enemy troops as possible. As soon as one of your soldiers gets to Sir William, you gain control of his army. Pull them back to your castle, trying not to get them in to combat straight away.
You'll need as many soldiers as you can for the next mission. If you have more than six months left on the timer, leave Sir William outside the keep for a while to allow a better economy standing for the next mission. If you have less than six months go ahead and get him inside the keep. Mission Complete!