Celtic Folly (Part 3) Help !!!

[gec]

I've been playing Part 3 for a while now (this level seems to go on forever), but I'm now stuck.

I'll try to describe where I'm at:

I got to the part where there are 4 Horseman Gems behind a glass, with a huge colored-glass mural behind them.

I went behind the curtains on the sides where I eventually found a "Pharaoh's Pillar".

I placed it on one of the receptacles (near the 4 gems), which "removed" the glass. After a few tries, I got the easiest gem (#4).

With 2 gems now in my possession, I left this room & went to the one right accross from it, where I had to use my 2 gems to get in.

That's the room I'm now stuck in...

I have searched this area thoroughly now, having pulled at least 4 levers, & pushed some wall buttons. (I'm not sure if it was more that 4 levers; I kind of lost track of it after a while.)

Anyway, I see 2 doors on the upper levels of this area; one directly ahead (from the entrance), & one to the right (after a small pool). But none of them seem to open after having pulled all those levers.

I also see another ledge on the left (as you come into this area). But it seems to be too far to be reached from anywhere.

What am I missing?
Which way am I supposed to proceed from here, & how?

Thank you all.