The top of the quarry walls and tower are still above the gas, as is the top of Lovis' tower, and a couple other places (in addition to the flying fortress platforms).
If you wait until near the end of Sentinel Island, and/or explored the fjords a bit before getting the battle tower, you could have taken out the first two guards (I did so at level 16 or 17, with help from my creature), but actually finishing the tower off would be difficult. It is meant to be done after getting the battle tower.
Ah thanks. I killed everyone outside, so at least I did that much already. How about that locked door at the base of Lovis' Tower? What was that about?
