"Instead, note the two lamps
on either side of you. They are blocked by white blocks, but you can move
them. To get the path open for both ones, push the top and bottom blocks of
each side towards the wall and then push the center one up or down. Don't
light the lamps yet though. Go south to the next room and do the same thing to
the lamps that should appear on both sides of you as well. Before you try to
do anything, let me explain this to you. To get to this next room, we are
going to have light the two lamps in the previous room and the two lamps in
this room. This is the reason that I told you to clear the path first.
When you're ready, head back north and quickly light the two lamps on either
side. Head through the south doorway on the double and light the two lamps on
either side of you as well. You have to do this fast or else the lamps before
you will go out. If you did correctly, the room will begin to shake for quite
a while before settling down. So what happened exactly? The only doorway other
than the one we entered from is the one leading east. Go through and enter a
large mesh room. The north end holds a communications plate. Avoid the
Wizzrobes and talk to Sahasrahla. He will tell you to light the four torches
and the way will open. We already did that, but where is the way?
Actually, did you notice the pit in the southeastern corner of this room?
That part wouldn't have been there. If you hadn't lighted the four torches,
this room would simply be a small corridor. Obviously, revealing that pit had
to do something so drop down and see where you land. You drop onto a platform.
There is one single chest here which, when opened, gives you the Big Key! Now,
you can either use the Magic Mirror or save and exit to get yourself back to
the dungeon entrance where we have some more stuff to do."
Here is the FAQ portion from the site listed earlier. It can explain this part better than I can.