Watch either of the two video walkthroughs below, depending on whether the party will be entering from the east or west entrances.
Entering from the west entrance offers a much shorter and clearer path to the Confidential Letter.
Avoid the night guards and wait for Fedel by the gazebo in the eastern garden area, which is circled on the map. Lurking in the room to the east of the gazebo is safest, as there is a guard who exits the castle's door before Fedel arrives.
Fedel will explain his quest and direct the Arisen to a location in the Cursewood where the lost confidential report was stolen by flying monsters.
At the marked location, the Arisen will find Fedel's destroyed cart and a gargoyle will be seen flying away to the northwest towards Soulflayer Canyon.
Reaching the correct ledge in Soulflayer Canyon can be a little tricky, but a pawn with full knowledge of this quest and Soulflayer Canyon will lead the Arisen to it.
From the west entrance, take a left at the passage fork.
After slaying the Cyclops on the bridge, continue straight until reaching a treasure chest on the left side and a steep downward grade on the right side.
Slide down the left side of this grade to the landing with two Godsthrone Blossoms (sliding down the right side leads to the main entrance) .
Slide down two more times toward the left to another area that may contain a snake and two spiders.
Collect some gold pouches, a Panacea and two Sconces on the left before encountering yet another downward slide.
Slide down the right side of this downward grade to a landing with more gold pouches, the fleeing Gargoyle, quest items scattered on the ground, and a chest containing a Steel Beak.
By forging the letter, the Arisen can obtain the gold and experience reward for the quest twice, first when given one Ser Gabrian near the entrance, and the second when given to Fedel.
Give either copy (real or forged) to either man to complete the quest.
Upon entering The Castle a short cutscene will draw attention to Ser Gabrian lurking behind a corner to the right of the entrance. This is the ONLY opportunity to speak to Ser Gabrian.
To collect both rewards Ser Gabrian must be given his copy first (real or forged), then Fedel the second one (real or forged).
The game's storyline will not be altered either way the Arisen handles this quest, but for the record it would appear that Fedel is correct about signs of a rebellion, whereas Ser Gabrian is attempting a cover-up by preventing news of it from reaching the duke.
The letter refers to the soldiers who later rebel at Windbluff Tower. As such, Fedel is correct in his suspicions. Ser Gabrian is almost certainly trying to intercept the letter and thus is in league with the treachery at Windbluff Tower.
Once Ser Gabrian is given the letter (or a forged copy of it), the quest will be marked as "Completed". However, the Arisen can still deliver the letter to Fedel upstairs in the castle. If the current active quest does not involve Fedel, he will have a blue question mark indicator over his head. He may speak as though the quest failed, but the "give quest item to NPC" popup menu should still appear, allowing the Arisen to turn in the letter and trigger the quest ending as usual.