Walkthrough for the "Tribes of the East" expansion.

I'm stuck on "Wisdom of the Earth," third zone. I've killed everyone I could and collected everything I needed. I went to the yellow tent, bought the machines, and visited the area behind the blue barrier, but the sorceress won't let me pass; it opens a battle zone. Yes, the sorceress in the hut mentioned a portal nearby and an artifact there, but I couldn't find any portal. Could that be the reason I can't proceed? Where is this portal? I've searched everywhere, but I can't find it.