I made it to the Black Gnarl but apparently I missed an item required to make the coffer / container. However, I cannot seem to go back. Leaving Fire Mountain, there is only one way to move, and you can only go forward one step. Same with leaving the Dark Tower--there is the way to the cliff (where you arrived), and the way that connects with Fire Mountain. I've tried using the "Whistle of Fiery Summons" from outside Dark Tower, both on the arrival side and the Fire Mountain side, but it says that the whistle does nothing. Any ideas?