Pass through the newly unlocked bridge and go south. Past the bridge search, the area for the chest and Boatman Coin. Only after you do that open the gate to the [Foundry].
[TIP: you have probably noticed that during this quest will be accompanied by Karn. You do not need to worry about his health. What is even more he'll make it easier for you to win most of the battles. The most bottom piece of information, however, is that she will help you access many of the inaccessible areas, and he will help you solve most of the puzzles.]