LAST EDITED ON Dec-19-01 AT 09:26 PM (Pacific)okay.
first off, this is a spoiler!
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~~regarding the idol~~
you found the dead t'rang, so you got over to the right side of the fort.
that side of the fort is where you need to look for the idol.
~the dead t'rang was at the northern end of that part of the fort.
~you must head south (staying inside the fort)
~you will kill some stuff and come to another smaller, room
~if you walk close to the walls inside that room, you will see some very very very very narrow stairs that wind upstairs.
~there are 2 sets of stairs
~take them up
~at the top of one of them you will see what you are looking for. it is very trynnie-looking. pick it up, and don't freak out. what happens next WILL freak you out, so don't freak out! ok? just go with it.
hopefully that will get you going. 
if you want an addy to a little walkthru, just ask and i will post it.
V***V
LAST EDITED ON Dec-21-01 AT 11:40 PM (Pacific)i went into my old saves and pulled up the one from just before i grabbed the Idol.
answer to your question:
NO, you don't go underground. 7/8 of all of this occurs at ground level. except for the very very last part.
first off, look at your auto map, kay?
at the southern end of the fort you see 2 deep blue squares, right? NOT THE BLUE RECTANGLE ON THE EASTERN SIDE!
the blue square that is in the south eastern corner is the one you want to get to.
~~>Idol Grabbing for Fun and Profit<~~
*stand next to the stinky, dead t'rang body and face south.
*there is a pool to your left.
*be careful there is a small trap that shoots one arrow as you leave and head south.
*the rooftop passage that leads to where you came up using the catapult is directly to your right. yes, you pass it again as you go south, looking for your idol.
*step into a small courtyard with grass and to your immediate left is another archway heading east.
*go thru the archway. the ground level drops a couple of feet.
*okay, you see 2 bldgs now. 
*to your left is the bldg with the lift that goes down into the trang place, right?
~~~>it's all glass and there's a funky dummy with a target on its chest dangling in the middle.
*ignore it.
*to your right is The Building, which on your auto-map has the 2 blue squares.
*walk the few steps and on your right (or south) is the entry way into the bldg.
*directly outside the door, to the left and right are little alcoves, with red lights in them.
~~~>look for goodies. sorry, i can't remember if anything is even there
but you will want to grab whatever you can, because, well.... it's just plain fun to do 
*enter the door.
*turn to the right. go all the way to the west wall, and turn around so you are facing north.
*see the kinda hidden passage? take it upstairs and look for stuff.
*go back down to where you were. pass by the entrance (so the door is to your left) and go to the eastern far wall.
*turn left and (just like on the other side~surprise!) there is a small, kinda hidden doorway.
*this is THE doorway you want. what you are searching for is in the room at the top!

lemme know if you still can't find it, kay?
V***V