Gwendylon : Last Skull - Level 7

After giving Nook his cap back he will tell you how the previous owner of the Tower had spoken of a hidden entrance. Stand between the torches in the room to open a secret passage, go in there and go down the ladder. The furthest room northwest holds an evil mage and his warrior accomplice. Draw out the warrior first, recharge, then head in and kill the mage. Grab the last skull. There is a key on the Mage, this allows you to loot the locked chests in this area.