The Nobleman Sword is a unique item quest and as such an be done whenevr you want. You must get all 3 items (dagger, amulet, sword) and sacrifice the dagger and amulet to get the sword. You must have high enough levels to beat the areas needed to get the items, but I did it before I ever even entered the castle. Also, enterig the astle should be almost the last thing you do in this area. The dagger is in the thieves guild, the sword is in Iona's dungeon (which can be entered due to a bug prior to entering the castle), and I dont recall where the amulet was right now. Check the links in the sticky and go to Kiya's Screenshot Graveyard. Great maps and walkthroughs for this quest!
