I'm at the save point before the spider ball guarian right now, though it's been several days since I played. Offhand, I can't think of how to describe the location, though I know I had to do a bunch of portal-hopping in the sky fortress (?) to get there.

If you're in the sky fortress area, jump into any dark portal you see and open as many doors as you can in that dimension, then come back through a light portal. There are walkthroughs all over the internet if you get really stuck.