Well, I just now finished the game for the first time, so I guess I'm not the most qualified person on the forum or anything, but I'll try to answer your question as best I can.
Once you're in the Asylum, you're pretty much stuck there for awhile. After the imprisonment, the dream, and finally the confrontation with Irenicus, you'll get a chance to leave, and you'll have two choices, go with Seamon Havarian on his ship or go through Irenicus's portal.
I went through the portal, so I have no idea where the other option gets you, although it seems like they could both end up in the same place. The Portal takes you to the underdark, and you'll be there for a long time, so I'd suggest that if you have anything to do back in Athkatla, I'd do it before entering the Asylum.
Eventually you'll get back to Athkatla, but not until after Chapter 6.
[This message has been edited by Avariel (edited 12-15-2000).]
Every man dies. Not every man really lives.
"Damn pointy-eared tree-hugging elf, prancing through the forests and whatnot!"