I've been doing the Firestorm Citadel quite a lot now, practicing taking out Lord Vanaduke before I attempt a Shadow Lair run. I see this button hidden under a few boxes:
Unfortunately, I forget exactly where this is, but as you can see (over the minimap), I'm on depth 27. This button does not have an obvious effect on the rest of the level. I've completed it with and without pushing it, and never noticed a difference.
What does this button do?