Knightly Catechism and How to Get the Flaming Sword
Pull the first two levers to open the first gate. The talking face will give you a book that you need to give to Sir Coffergus in the Eyrie to translate. The translation contains the answer to the puzzle of the second talking face on the same level (behind a secret door).
After stepping on the magic water tile to become baptized, you can simply click on the wall next to the backer perk chests and it will disappear.
The backer perk chest contain items that are as good as the Flaming Sword, by the way. You need 7 or 8 perk keys to open them all. The last one in the NW is empty, so you really need only 7.
Speaking of the Eyrie, put on the Avian Helmet and then sleep on the round tile in the NE corner of that map. Prepare to be amazed.