This is the forum for helping you out if you're stuck or if you want to discuss the nitty gritty details of the dungeons within Mount Grimrock. Warning: forum contains spoilers!
That portal needs to be activated first. Investigate level 12 and 13 to do that.
Thank you.
I spent 2 hours investigating every wall of Level 12 and 13 just now. But still, I can't find the button or trap that make the portal activated. It almost driving me mad. Can you tell me more about the portal? Thx.