Unlocks: Eruptor’s “Mega Magma Balls” Ability
Immediately after lowering the first gate, you’ll come to this wall that must be destroyed with a cannon. Use your pick axe to make a path through the rocks to the south and reach the cannon.
You’ll have to destroy the wall to progress in the level, but fire it a second time to destroy another wall.
Walk down the tunnel past where the second wall stood to find a Soul Gem.