I've thried the spoiler walkthrough, but it refers to a "narrow ramp" that I walk up to get to the southeast tower in Marten's Bluff to recover the Stone Idol. I've searched all over but cannot find such a ramp, or any way to access that part of the castle.

Spoil me please!

Are you in the castle?

I'm strolling about in the courtyard I think. I saw the dead T'Rang and I can mess with the control panels and such. There's that large double door on the south wall, but that won't open. Is there a way in the castle itself without going down to the lower levels?

You need to go through the mines first, the alternative way to get to the bluff, really easy too. Just go to the north eastern most part of the swamp to start out. It is actually ass backwards, but that is the way I did it as well. Then you will see a lever inside that lets that big red door go away.